My reasons for this is that, underwater if you have a shark in front of you, you can swim around it, rather then getting boxed in by sharks which.. is quite impossible. I'd understand not being able to walk through a jelly cube, as that does take up a large space, but you should be able to just, swim past leaches, gray oozes, sea slugs. Due to the fact that you are swimming not treading along the bottom.
I'm not sure this is even possible in NWN unfortunately. Even invisible creatures / ghosts etc. are unfortunately substantial when it comes to pathfinding on the walk mesh.
Unless you are some sort of fish-creature the shark is easily going to outmaneuver your fullplate fighter underwater. And I recall certain aquatic subraces receiving perma-haste underwater.