I believe what you are reffering to is called physics. Good luck jumping underwater in real life.
The other games had underwater combat. Pre-Oblivion, there was a waterwalking spell. Oblivion had jumping while in water as a perk for mastering acrobatics. I don't bother with the water in Daggerfall yet, so can't say much about that one. I'll be in a boat... Not singing I'm On a Boat. Nope... I will sing it anyways.
How is combat underwater anymore unrealistic than riding a dragon to someplace? How about riding a horse and the horse being able to scale ledges? Let's not forget the fact that realism shouldn't make something taboo. If amphibious, tree born, lizard people are okay and a horse being able to scale more than one ledge is okay, then I don't know how making swords and weapons weaker when used underwater would be so unrealistic.