You really shouldn't be able to shoot high-velocity kinetic weapons underwater. Water doesn't compress well and would stop any round within a few meters. Hey, I saw it on "Mythbusters" so it has to be true.
I can see allowing lasers and missle launchers under water, but only if the lasers are tuned to blue-green, I read that one in the old "Boat Wars" game by Steve Jackson, so it has to be true, too.
Otherwise, you can just say the fluid isn't water, just some non-energy absorbing fluid with water mimicing properties.