Actors ignore vertical axis when deciding to attack
Turn off collision and go hover over some water. A bunch of slaughter-fish will huddle up below you and go through their attacking motions over and over again even though there is no way they can get to you.
In Morrowind, the slaughter-fish would not try to attack you if they can not reach you. In fact, if they cannot reach you (e.g. you are standing on land), they will ignore you completely. In OpenMW, slaugter-fish and other swimming creatures will gather near the shore if the player is close to the water. I don't think this behavior is bad, but it does seem like they should be smarter about deciding when to attack.
(RM-1335 from redmine: created on 2014-05-18 by Alex McKibben, , closed on 2014-05-26 by Oleje Cheko)