Cats will eventually drown when in any sort of water, despite being at the surface and not visually submerged. This also seems to affect other small mobs, such as silverfish, baby wolves, etc.

• Spawn a cat or any small mob (through spawn eggs or /summon) in any body of water
• If it stays in the water, it will eventually take drowning damage and die, even when its at the surface (not submerged)
  • Using /summon seems to start drowning the mobs immediately
• Mobs affected include:
  • Cats and baby cats
  • Baby ocelots
  • Baby chickens
  • Baby pigs
  • Baby wolves
  • Parrots
  • Silverfish
  • Endermites