Yes, that's true, and might be a partial explanation. However, the same can be said for lots of other species too. For examples, sea-based turtles. As far as I know, no-one is saying that they are fundamentally unchanged for millions of years, as they do for crocodiles.
To be clear, I don't want anyone thinking I don't believe in evolution! I do.
Okay accepted it is a partial explanation. Evolution is based on random mutation and if you accept that, then the mutation needs to coincide with a favourable change in the environment or habitat, for a new strain to evolve. So whilst Sea Turtles may share many of the characteristics of the crocodile in terms of longevity and infant mortality, their environment is much more diverse, given the vast distances they are known to travel. The evidence for evolution also focuses on the development of the hard shell. Clearly without this defence sea turtles were vulnerable to predators in a way that ancient crocodiles were not.