Here is how I cast underwater.
Let tank/pet take agro.
Swim straight up using first person view.
Turn to see mob, watch my motion, and cast.
If I'm soloing in Kedge,(Mage) and I pull more than one, I swim straight for a corner, and turn to see the mob. If I already have my back to the wall, mobs dont push me around, and I can cast happily until pet comes to the rescue, or I blast the hell out of the mob.
Mobs don't push you up or down, so if your in a tunnel, you'll slowing sink, but not enough to interupt casting.
Happy hunting!