The average age of cruise passengers overall has fallen to its lowest in 20 years, and a record 1.7 million UK holidaymakers opted to take to the seas in 2016, according to the Cruise Lines International Association (CLIA).
Some operators say they need more ships for 2018 just to cope with demand - so why has this type of holiday become so popular?
Janet and Tom Doyle, both aged 59, from Runcorn, in Cheshire, recently took their first cruise holiday as a family, with sons Jason, 21 and Christopher, eight.
"Tom never wanted to do a cruise," said Mrs Doyle. "But last year we went to Florida with the kids for three weeks and there was so much driving."
"I thought I would feel like a captive audience," said Mr Doyle. "I thought I would end up stuck on board but didn't realise the facilities modern cruise ships have."
In April, new cruise ship the Majestic Princess, which will be based in Shanghai, set sail. The Doyles were among a number of families on board its inaugural cruise from Rome.
"There was a lovely mix of ages on the Majestic and in the kids' club Chris was talking to a boy from Beijing. He'd been learning about China at school so it was great for him," Mr Doyle said.
"There's a perception the people on board are 10 years older than us but I saw several big groups of people in their late 20s," he added.
"The world's getting smaller - who'd have thought 20 years ago, a normal, working-class man like me could afford to do this?"
Adam Coulter - editor of www.cruisecritic.co.uk
There are several reasons that have come together and caused this changing market.
You have a vast choice of ships which appeal to family groups, as well as millennials. I've just been on a two-day cruise to France and I've never seen such a young demographic. There were hen parties, 40th birthday groups, multi-generational and small family groups.
There's also a growth in theme cruising. Anchored, a ship-based dance festival, took place in June and was headlined by Tinie Tempah. These are common currency in the US - from Star Trek to music themes - but are just starting to really take off here.
Cruises are good value. If you're a family, the kids' clubs are included, as is all your food and entertainment. The childcare really is of a high standard and ships are so safe.
And cruise lines have made massive strides in technology and internet connectivity, which appeal to a younger demographic. Ships have made their wi-fi cheaper and quicker as they've realised people want to post on Facebook, Instagram, Twitter. It's all about the experience.
Two 20-somethings on board the Majestic were Sophie Ortyl, 28 and her boyfriend, 25-year-old Joe Campey, both from Crawley in Sussex.
"This was my 11th cruise," Ms Ortyl said proudly.
"My nan paid for the whole family to go on a cruise when I was 16, for her wedding anniversary, and I thought it would be all old people. But there was so much going on, the people were fun and the food and drinks were amazing."
Now hooked on cruising, Ms Ortyl has done four maiden voyages and has booked to go on Royal Caribbean's new ship, Symphony of the Seas, next year with her boyfriend.
"The food on the ships is amazing, the best food I've had. If I hadn't have met Sophie, I'd never have done a cruise," said Mr Campey.
Certainly it's a type of break that some holidaymakers would never consider; suspicious about the company they'd be forced to keep and unwilling to sacrifice the freedom to make their own itinerary.
Some might also cite the risk of mass outbreaks of sickness and diarrhoea as a major concern. In the past, cruise companies have paid out thousands in compensation to passengers whose holidays were ruined.
Others have ethical concerns about the poor pay and conditions some crew members face, most working long hours, with their take-home pay dependent on commission or tips.
Keeping it all shipshape - Francesca Maglione, on board events manager for MSC Meraviglia
"Every day is a great surprise for me. I start early and the day can take many unexpected turns. I organise all of the events that take place on board... parties, meetings, conferences, weddings, proposals, anniversaries and much more.
"I'm an organisational madwoman, as you'd expect, and without a doubt my highlighters, coloured pens and markers are among my prized possessions!
"The best part of this job on board is that you are working as part of a team, which often feels more like family, with a mix of colleagues of many nationalities.
"My most memorable couple so far was from Portugal. The ceremony was a surprise for the bride and was very romantic. I felt proud we had a chance to share this together so by the end of the ceremony all of us were crying, officers included.
"[It can be hard] when I reach the end of my emotional reserves after a long day but thanks to my experience, I know how to manage this."
Of course would be misleading to say cruise ships are packed full of under 25s and family groups - the average age of a cruise passenger is 55 - but companies are working hard to expand into these markets.
With many UK schools breaking up for the holidays, cruise operators are looking at record numbers of families on board.
Princess Cruises, part of the US-owned Carnival group, is open about its plans to attract more families with a complete overhaul of its kids' clubs and a partnership with the Discovery Channel.
On and off its ships there are further options for families including tailored shore excursions for parents and children, onboard sports and activities such as kids' dance classes, and family movie nights via its top-deck cinema screen.
As a result, Princess has reported a drop in the age of its guests. Next summer there will be almost twice as many UK under 18s on its main Southampton-based ship than there were two years ago. And the number of 35-44 year olds - many of them parents - has doubled.
Tony Roberts, Princess Cruises' vice president UK and Europe, said: "Our core guest is still over 50 and we would never shy away from ensuring we cater for that audience.
"But families and younger guests are unquestionably an ever-growing market for us."
Edie and Eloise, eight-year-old twins from landlocked Nottinghamshire, were on the Majestic Princess' inaugural cruise - also their first - from Rome.
"We loved the ice cream and the swimming pools. And the shows were really, really good, especially the Unexpected Boys and the Beatlemaniacs.
"We dressed up for dinner every night and had our own kids' menu - we could have a special pudding and if you didn't like it, you could have a different one.
"Kids' club was brilliant. On the last night, we didn't go to the restaurant with mummy and babcia (grandma) because we had a pyjama party and wanted to play. And the helpers were all very kind."
In the growing market of family cruises, Royal Caribbean has established a firm foothold. Last year saw the company launch the biggest passenger ship in the world - Harmony of the Seas - equipped with the Ultimate Abyss - a slide that drops 10 decks - zip wires, 18 restaurants and 32 bars.
"The average age of our passengers is 49," said Ben Bouldin, managing director UK & Ireland, for Royal Caribbean.
"By the nature of our ships and our innovation, we lend ourselves to a younger, more family-driven market.
"We're not only focused on kids though, as they can only really go in school holidays, and we've got lots of other demographic groups who travel at different times," he added.
How much ice cream? Harmony of the Seas in numbers
- At 1,188ft (361m), it is longer than the Eiffel Tower is tall
- The ship accommodates 6,687 guests and carries 2,193 international crew members
- Fifty tonnes of ice cubes made on board each day
- Passengers make their way through 2,500lb (1,134kg) of fresh salmon and 2,100lb (953kg) of lobster tail on an average seven-night cruise
- About 100 gallons (450 litres) of chocolate ice cream are consumed per seven-night cruise
- There are 3,415 pieces of sports equipment on board, including 185 pairs of ice skates, 130 golf clubs and 31 surfboards
- The ship's crew and passengers use 1,800 tonnes of fresh water every 24 hours
- There are 10,587 plants and 52 trees in the Central Park neighbourhood
"The biggest challenge we have is dispelling what many people think cruising is about. There are about 1.7 million people in the UK who cruise, yet there are 40 million holidaymakers a year.
"We're really competing against land-based holidays to get more customers.
"Due to macro-political and economic issues in certain areas of Europe, some places aren't as accessible as they were, prices are being driven up and cruising is much better value."
John and Trish Smith, both 50 and from Horwich in Lancashire, have been on dozens of cruises.
The couple - John a custody sergeant and Trish who works as a local council safeguarding administrator - used to go a static caravan park in France for all their holidays. But when they took their first cruise with P&O in 2001, with a group of 38 friends and family, they were hooked.
"We've got four kids and didn't think we could afford a cruise," said Trish. "But we found a good deal, with a four-berth cabin, and never looked back. It was cheaper than going on an all-inclusive package holiday."
The pair have travelled all over the world on cruise ships, their latest a trip around the Med - this time without the children.
Mr Smith said: "We've just sold our touring caravan but we love both types of holiday. What's so good about this is you don't have to pack and unpack and get to see lots of different places.
"On the first cruise we were on, there were some lads fighting in the nightclub and they got turfed off at the next stop. There was also a couple in their 80s who got sent off because they'd been stealing cutlery and stashing it in their cabin."
Mrs Smith added: "We've been on cruises with a man overboard, been through a really bad storm and experienced a medical emergency where a woman needed a blood transfusion and loads of people came forward to help.
"There's a preconception that cruising is stiff and starchy but it's not like that any more. You've just got to do your research and find the cruise that's right for you - there really is one for everybody."