Underwater combat is an interesting proposition, but is a very real possibility on certain planetoids in the Solar System (Ceres, Europa) or even exoplanets. Melee combat is slower and more exhausting, physical projectiles normally move slower, which reduces their effective range and accuracy, and the additional recoil can damage weapons. Beam weapons may entirely not function in water, or have greatly reduced range due to much higher diffusion. Grenades and explosives are very powerful, but harder to use underwater. But, for those who need to engage underwater, there are options based on classical technology which may be functional.
Harpoon Gun: A modern version of of a historical weapon, the Harpoon Gun is rare off aquatic planetoids like Ceres and Europa, though occasionally seen among Gatecrashers on aquatic planets. It has a basic function, firing a small spear or harpoon which is engineered to travel through the water. They operate of a canister of compressed gas, which is good for 100 shots and [Trivial] to replace. The Harpoon Gun has a low rate of fire, but can shoot a variety of ammunition (see below) reliably through the water without cost of effectiveness. A modern harpoon gun comes equipped with a Smartlink and a Safety System. [Moderate]
Harpoon Gun SS Ammo 1
Harpoon Ammo (Price for 20):
Harpoon: Standard harpoon, which is designed to be fired underwater. AP -6, 2d10+4 DV [Trivial]
Barbed: Sharply barbed harpoon which inflicts more harm, but less penetration. AP -2, 2d10+7 DV [Trivial]
Injector: Hypodermic harpoon, designed to deliver a payload of drugs on impact. AP -5, Half DV. [Trivial], plus payload cost
Limpet: A specialized harpoon which is designed to carry a minigrenade when fired, which then detonates as normal. It's AP and DV are as the grenade. Halves the maximum range. [Trivial], plus Grenade cost
Diamond Harpoon: Variant of the harpoon edged in industrial diamond for better armor penetration, but imparts slightly less force to the target. AP -10, 2d10+2 DV. [Low]
Net: Harpoon Gun round which is actually an expanding net designed to entangle the target. Target hit with the net is entangled and takes a -30 on all physical actions and mostly immobilized unless they can make a COO or SOM x3 test to escape. Large targets only take -10. Reduces range by 50%.
Pronged: A harpoon which has it's normal head replaced with electroshock prongs, ideal for incapacitation, but reduced maximum range by 10%. AP -3, 2d10+3+Shock DV. [Low]