Fighting in an underwater environment will be a serious challenge. Until proper diving suits are developed, we'll have to rely on oxygen tanks or Stimms to stay conscious. But that is only one of the problems; we'll also be facing painful penalties to all combat skills.
>Melee Combat. With hampered movement rates and energy regeneration, only the very best warriors will be able to go toe-to-toe with aquatic enemies, who operate on their own turf, and thus will outspeed and outdodge us.
>Throwing Weapons. Throwing is penalized as well, but these can work surprisingly well as middle-ground backup, as there will be time to replenish our Energy. Especially interesting will be weapons that use half throwing and half melee skill, due to their good attack ratings.
>Ranged Combat. This will be the most advantageous, but hampered by the sparsity of good underwater guns. Looks like Crossbows and Harpoon Guns are going to be the mainstay weaponry.