South West scuba experts are worried inexperienced divers who have only received basic training abroad may get into trouble in the region's waters.
Those who learn to dive abroad want to continue back home
Scuba diving is growing rapidly, helped by the recent high-profile sinking of Scylla back in March.
But it is feared that novices accustomed to tropical waters could be shocked by the cold and turbulent conditions in English waters.
They also fear divers could snag themselves or equipment on wrecks.
Experts in the industry say overconfident "holiday divers" are a danger to themselves and others unless they get local guides and training.
Gary Fox, from St Keverne in Cornwall, leads expeditions around the Lizard coast.
He said: "We've seen a big increase in people who have gone abroad to do a diving course where, without a doubt, it's a lot easier.
"I think people are just pushed through to get a certification.
"We always ask if they have dived in England. If they say no, then we don't think they're capable without training."
Brixham Coastguard Stuart Elliman said: "It's a three-dimensional game, whereas most other water activities are two-dimensional. They stay mostly on the surface.
"But this sees people going down into the cold, wet and damp, it's usually dark, and orientation can be difficult. There are all kinds of hazards."
Organisers of the Scylla's sinking, which included the National Marine Aquarium in Plymouth, say they have built extra large access holes and stripped the wreck of anything hazardous to try to minimise the risks involved at the vessel.