Despite their semi-aquatic existence, hippos are actually not great swimmers. They don’t have streamlined bodies or flippers, and although their toes are webbed, their legs are rather short and fat. Instead, they mostly move with a sort of slow-motion gallop along the riverbed. In order for this to work, they have to be denser than water.
Most mammals are naturally buoyant, but hippos have especially dense bones to help them stay on the bottom. Seawater is about 2.5 per cent denser than fresh water, but the extra buoyancy this provides isn’t enough to offset the weight of a hippo, and it will still sink in the sea.
And because the buoyancy is always equal to the weight of the volume of water that the hippo displaces, it remains the same regardless of depth. So once the hippo starts sinking, it’s doomed to make a one-way trip to the ocean floor.