Now consider any number of munitions attached to the spear projectile.
Concussion, neurotoxin, tranq, stun grenades, EMP, frag.
Subsonic rounds tend not to break up as badly and remain lethal out to two squares. Now adapt that concept with a subsonic gyrojet smart round built for underwater use.
A fusion block doesn't care what medium it is detonated in, it will simply annihilate whatever is nearby. And by nearby they mean nearly 40 feet. Now if it explodes under you, there is an 80 foot cavity that is going to be filled quickly by whatever is above the expanding steam bubble.
Any size warhead can be designed to fit into a torpedo of appropriate size. You just reduce the speed and effective range against moving targets. So RPG's can still be a thing if you specify the munition was designed for use underwater. With explosives underwater, proximity fuses are a thing.
Most above water based weapons will work, albeit less effectively, but perhaps only at point blank ranges. So like laser welder ranges for most energy weapons. Supersonic rounds break apart, quickly, making for a very short ranged flechette round, but designing super-cavitating rounds make them effective.
Basically, any type of weapon that you want to use underwater, CAN be used as a dedicated underwater weapon by a sufficiently advanced society .
