I've noticed this only in this snapshot, but may be in other snapshots. Many mobs move significantly slower in water than they do in the current version, 1.7.4. I've even tried with speed II on them, and still the same slow speed. I've tried it with all hostile mobs that could swim, with speed II, and noticed almost no speed difference. I then tried it in the current version, and they went so much faster, even without speed II.
To reproduce:
1.) Find a body of water(River or lake) with land on both sides within the mob's search radius
2.) Spawn a mob on one end of the lake, give it speed II, and go into survival at the other end
3.) Notice the mob get across VERY slowly.
It's the little bugs like this that I tend to notice...