It seems that crocodiles intentionally will eat stones or rocks before diving into the water. It has been thought that one reason is to help with digestion. Rocks that have been in the digestive system of an animal are called gastroliths, which literally means "stomach stones." Rocks in a crocodile's stomach might help crush and grind food especially for crocodiles who eat whole prey, particularly animals with shells and tough bones. A gastrolith can remain inside the stomach for years. Some scientists think they swallow the stones to increase their weight and to feel fuller.
For years, it was hypothesized that eating stones might also help a crocodile stay under the water longer and to dive deeper. Many crocodilians like to float in the water with just their eyes and nostrils showing so they can ambush their prey. It was thought that a stomach full of rocks might help the crocodile keep their bodies under the water and out of sight.
New research shows that this hypothesis might be wrong! Recent research by a paleontologist named Don Henderson has shown that for the rocks to help stabilize the buoyancy of the crocodile's body, the rocks would have to account for at least 6% of the body mass of the crocodile. They have now measured it, and the rocks only account for about 2% of body mass. Below 6%, the filling and emptying of the lungs have a much greater effect on the buoyancy of the crocodile than the stones. However, a low number of rocks might keep the crocodile from rolling from side to side. Also, with fewer rocks, they might not have helped grind up their food either.