Two divers showed signs of the bends minutes after surfacing too rapidly in Caernarfon Bay and had to be airlifted to safety, say coastguards.
The pair lost control of their buoyancy as they rose from a wreck 25m (82ft) down, said Holyhead coastguard.
An RAF Valley rescue helicopter took them from their dive boat to a decompression unit on the Wirral.
A coastguard member said: "Because they were having symptoms so quickly, we got them there pretty much straight away."
The officer said the pair had been diving the wreck Segonium, a mussel dredger that sank in 1984, when the call came in at around 1415 BST on Sunday.
He added: "They knew themselves they came up too quickly, their dive computer probably told them.
"They lost control of their buoyancy and could not stop their ascent. It's quite easily done."
In an unrelated incident earlier in the day, Barmouth RNLI inshore lifeboat was called to rescue a small dinghy which was still under power from its outboard engine after the two people on board had fallen out.
Coastguards said the couple who had been on board raised the alarm after swimming back to shore when they were tipped out of the 2.5m (8ft) craft as they went through surf at Talybont.
The boat was half a mile out and heading out to sea at around five knots with a full tank of fuel when the alarm was raised.
A Milford Haven coastguard spokeswoman said: "It's not entirely clear how they came to be off the vessel in the first place.
"They said they got caught in the surf and another wave came along and tipped the vessel and they fell out.
"These vessels do have "kill cords" and the idea is that this is attached to driver and the cord cuts the engine. This driver was not wearing it."
She added that the inshore lifeboat crew, who were on exercise in the area, caught up with the boat within 10 minutes and brought it back to the owners.