Boat owners in Devon and Cornwall have been asked to watch out for basking sharks by an animal charity.
The sharks come every summer to feed on plankton
The Shark Trust has issued a new code of conduct for people at sea after several cases of sharks being badly hurt off the South West coast.
The trust said boat propellers and personal water craft could inflict severe damage to fins and bodies.
It is asking boat users to act considerately and switch their engines to neutral if they see any sharks.
The creatures, which can grow to be longer than a double-decker bus, appear off the region's coast every summer when good weather increases the surface temperature of the sea.
The sharks come to feed on plankton and sightings have been increasing for 10 years.
Stephen Tooke, a former harbour master at Salcombe in Devon, says the creatures attract people like "bees to a honey pot".
He told BBC News: "They just want to go over and look, but sadly a lot of people just don't know how to behave."
Ali Hood from the Sharks Trust said boat owners should be cautious if they spotted any sharks.
"The concern is for every one you see on the surface, there could be a number below it that boats could well encounter."