Turtles do not seem to lay their eggs. I breed some (several times), sometimes where they appear to spawn, and once they're done breeding, they both swim off, even though it would appear they spawned on the island I had just breed them only blocks away from. Even following the one with the eggs produces no results, that turtle will simply do turtle like things, such as swimming around or in circles, never actually going to a beach to lay the eggs. 
Summary
The turtles will not lay their eggs even after they've been breed, seemingly no matter how far OR close their home beach is.