I made a customized world with sea level at y=10 and there was a slime at Y=39 in deep ocean. They only spawn at Y=39, not 1 block higher, not 1 block lower.
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http://minecraft.gamepedia.com/Slime
Slimes spawn throughout the world (except mushroom islands) below layer 40, but only in certain chunks, 1 in 10 of all chunks.
I just watched Etho Plays Minecraft Episode 309 today and figured out that's been in the game a long time. For some reason, I thought they only spawned at Y=14 or lower.
Intentional. Slimes spawn below Y=40.