A friend is going to run a FATE mecha game where all the action happens deep underwater. Since many traditional armaments don't work for that without some alterations, I decided to brainstorm a few variations that would be suitable. It's naturally very important to give them names that are suitable for shouting out loud.
Harpoon Machinegun
Harpoons are a staple in the undersea warfare, and it would make sense to crossbreed one with the trusty Vulcan cannon. You'd need some mechanism for attaching strands to harpoons as they leave the firing mechanism.
Supercavitation Crusher
Supercavitation is a real thing employed by mantis shrimp in nature to aid crushing hard shells of its pray by adding secondary impacts to the physical one. Supercavitation Crusher would be a blunt rod of metal (possibly square or hexagonal - you don't want it sharp and it wouldn't last anyway) with a suitable acceleration mechanism. In the depth of a few kilometers you don't need to crush someone's armor entirely - just adding a good crack will ensure Mother Nature does the rest.
Microwave Lancer
Water dampens radiation a lot, preventing the use of many traditional beam weapons, but that just means you need to get close in to do the job. Microwave Lancer requires you to close in with the enemy before frying them in a burst of radiation.
Railgun Spike
Somewhat related to Supercavitation Crusher above, this is more similar to your garden variety pile bunker. To spice it up, acceleration mechanism doesn't use gunpowder, but magnetic linear acceleration instead, driving the piston along two electrified rails. For extra convenience, you can reverse the polarity to retract the spike. Try not to get any sea water inside or it will short out!
Predator Claw
What's the better way to rub the inevitable into the enemy than grasping them in a huge sharp claw before crunching their robot in half? I don't think science is ready to answer this question yet. Don't get your fingers in it.
Drill Torpedo Punch
So rocket punches don't work, see if I care. Just fit a forearm with a hidden torpedo engine and get to work. Note that these should probably be wire guided if you don't want one zooming out of your pitiful radio contact range and having to do without an arm for the rest of the trip. Since impact alone won't be much, slide a drill head in front of the fist - if you can connect it to the torpedo engine somehow, all the better.
Chakram Cutter
Another close range weapon, Chakram Cutter rests in two semi-circular toothed halves on both forearms when not deployed (you can use the edges for hand to hand brawling, if necessary), which connect into a large arm-mounted buzzsaw . For bonus, you can use it like a shield to deflect enemy projectiles and harpoons.
Magnetic Anchor
There are other fun things to do than just kill someone. If you're fighting close enough to the bottom of the ocean and there are things to entangle wires into, affix one end of a very strong metal wire there, set the other one into a powerful electromagnet and fire it at the enemy (both ends should be fired at once for better effect from a sort of double combination launcher). Now it's their job to figure out how to unattach the electromagnet from their metal armor while their mobility is reduced.
Vapor Resonator
Suddenly turning water into a sea of bubbles wreaks havoc with floating objects, as they are suddenly dragged down. A ranged weapon that manages to achieve this effect in beam-like fashion won't kill anyone, but might take someone out of the action for a few critical moments.
Impact Hammer
A Chinese meteor hammer version rather than a Western warhammer, it is simply a very large ball of iron with optional spikes, with a bunch of powerful torpedo engines set up in every which direction, connected to the parent robot with a chain housing control wires. Just clobber the target with it until they say "enough" or you can't identify the model anymore.
