Yes they do, but a lock is made to keep honest people honest. Guess a lock cap will give someone a second thought, but keep in mind they can also do damage getting it off. I think the best thing is a camera
If you google boat locking gas cap, it comes up with several styles
Ayuh,...... If I was gonna steal yer gas, I'd unhook the fuel line at the motor, 'n pump it out,...
A lockin' gas cap wouldn't even slow me down,....
Fortunately, I ain't a thief,.....