One of the problems with underwater combat is that it adds another axis to the system.
On ground you only really have to worry about the X and Y axis, sure sometimes there's ledges an rocks but the target itself is not really moving in the Z-axis (altitude).

Anyone that played Aion and experience the flight combat would know that this is extremely disadvantageous for melee who have short attack range and now having to worry about their target now being able to escape by going over or under them.

To add some realism for underwater... people swim right? The movement should decreased effectively making you pretty much shark bait. And I wouldn't even think about casting fire or thunder magic underwater.