I was testing out minecraft 1.4.3 pre release when I found clay in a little pond, I dug up the clay and fell in to the hole to get it, and I couldnt swim back up, it was a 2 block deep hole I fell into, there was no ice it was a grassy biome, my keybored was working cause I tested this out on other water and I could swim fine, I was holding the spacebar and it wouldnt go up so I drowned, its not this way with all water though, only when I stand on clay IN water