Swimmers and boat users are being asked to not disturb basking sharks as part of a new code of conduct.
The Plymouth National Marine Aquarium and the Shark Trust say boat owners should also restrict speeds when near the animals and avoid large groups of the creatures.
It follows concerns about incidents in which the sharks have been disturbed.
Basking sharks, often seen off Cornwall and Devon in the summer, can grow up to 10m (33ft) long and weigh seven tonnes.
They feed on plankton and, while not normally referred to as dangerous, their sheer size and power make them creatures that need to be treated with respect and caution.
The trust said this had been reflected in the code.
It said: "In the excitement of observing these magnificent creatures, there are often reports of sharks being struck by boats or other craft, as well as being disturbed by swimmers and other water users."