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.
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The report you have submitted is working as intended, mechanics of the game may change between updates. Things such as graphics, sounds, world creation, biomes, redstone, villagers, and animals may not work the same in current versions.
For more information see: Full Version History or Snapshot Version History.
WAI. This is part of the new Mob AI.