Slaughterfish, which are the only reason to fight under water, can be killed from the surface with a bow.
However, underwater melee was in earlier games. It is probably because of the lack of underwater content in this game so far (I am hoping anyway), there was no reason to put it in.
Just because something is less present than in previous games is no reason not to include mechanics.
Dealing with water is just annoying now.
Yes, you can cast a cloak, yes you can kill on land.
Does not change the fact that its cumbersome, annoying and irritating that I cant swing a dagger underwater.
Same goes for waterwalking.
Zero good reason for its omission, only makes the game more shallow.