Army bomb disposal teams are regularly being called out when magnet fishers have found artillery at the bottom of canals and river beds, the Ministry of Defence has told the BBC.
In some cases, live ammunition - such as grenades - has been found.
Magnet fishing - trawling for metal objects in the water - is a growing pastime, with many people doing it to clean up the waterways.
But the Canal and Rivers Trust called it "dangerous".
The Ministry of Defence told the BBC's Victoria Derbyshire programme it advises the public "that if they inadvertently disturb what they believe to be live ordnance, they contact their local police force as a matter of urgency".
Callum, 15, from Leicestershire - whose surname the BBC has chosen not to use - said he had found guns, safes and a live hand grenade in canals.
"It's very exciting, because you wonder what views you'll get," he says, referring to his YouTube channel.
While the BBC filmed him and his two uncles, it took just 10 minutes before they found a knife in the canal.
They also discovered scrap iron, which they gave to a scrap dealer.
Some people choose to sell the scrap metal they find.
Some magnet fishers have been praised for cleaning up the environment.
Callum's uncle, James, originally took his nephew out to stop him playing video games.
He defended what they were doing - including the cost to the taxpayer when bombs or other artillery are found and the authorities have to be called.
"The bombs are probably safer being out (of) the canal, and being in the hands of the bomb squad," he said.
But the Canal and Rivers Trust has warned people to be careful when removing objects that have been underwater for a long time, calling it "dangerous".
Gillian Renshaw, from the trust, said it had seen a spike in reports of magnet fishing as it continued to grow in popularity on social media.
Two men drowned while magnet fishing in Huddersfield, after being pulled into the water.
The trust also points to a by-law that prohibits items being taken out of the public canals it owns, which can lead to a £25 fine - though that is thought to be rarely enforced.
It is not a criminal offence to magnet fish on private land, with permission.
Watch the BBC's Victoria Derbyshire programme on weekdays between 09:00 and 11:00 BST on BBC Two and the BBC News Channel in the UK and on iPlayer afterwards.