X-COM: Terror from the Deep/Equipment

Infantry Weapons
When starting the game, sell your dart guns and their ammunition. Replace them with Gas Cannons and high-explosive ammunition, which are more effective in the early game. You will also want to sell the torpedo launcher once you get the replacement equipment.

Of the weapon sets, the sonic-based weapons are the most powerful. This permits you to skip Gauss research if you can properly equip your team.

Sub Armaments
While the gas cannon and AJAX torpedo work well for early engagements, they are rather weak weapons. To get the more advanced weapons, you will need to research the appropriate technology. For the Gauss cannon, you will need to research all the infantry gauss weapons and their ammunition. For the Sonic Oscillator, you will need to retrieve and research all three sonic weapons and their ammo as well. The P.W.T. Cannon is available once you research the Disruptor Pulse Launcher.

Note that the more powerful weapons are likely to destroy craft rather than crashing them. If you are interested in salvage, you should still keep the weaker weapons in reserve.

SWS
The Submersible Weapon Systems are the equivalent of tanks found in the previous X-Com game. They are strong and resistant to shots, but are expensive and take the place of four aquanauts. It is a matter of personal preference on whether to deploy one, but it is still useful in the early game if you haven't yet obtained armor.