the baby zombies can move in water but very slowly
it seem to be that the movement speed of mobs is decrease in water.
because baby zombies are faster, the can swim slowly and other mobs can't move because they are to slow
mobs can't swim AND can't jump on a 1 block higher level
somtimes mobs still glitch through (wood)fences in multiplayer when i run a own server on my desktop PC
i guess that the bug orrcures if nobody is on the server or then the first player join / leave the minecraft server
if u need a savegame + server konfiguration pls tell me
actually we are using minecraft_server.1.8.2-pre1.jar for the server and the client(s)