Prior to the 1.8 official release, Silverfish, Magma Cubes, Endermites, and Slimes would sink to the bottom of water pools. After 1.8 was released, all of these mobs are now capable of swimming.

I'm not completely sure whether this is a bug or intended...

To see what I mean, play a creative world in 1.7 and spawn Silverfish/Slimes inside a pool of water. They should stay at the bottom of the pool. Then, update the world to 1.8, and the Silverfish/Slimes will rise to the surface and swim like any other mob.