This is the standard diving suit in Terror from the Deep, worn by any aquanaut not wearing something else. It does not need to be bought or manufactured. (Although how X-Com gets free scuba gear for all its troops is quite a question! Maybe it's included in the $40,000 hiring fee...) It is unlikely to help against the alien's sonic weaponry, but it may stop a stray dart or harpoon. Being idiots, your motley crew will insist on wearing this suit even on land-based terror missions. This causes them to lose any gain in mobility and dexterity that one would normally expect from fighting on the surface, instead of 1000 feet below sea level. Presumably the diving weights contribute to the minor protection the suit has to offer.
A Diving Suit does not provide any damage resistances, but has a 60% weakness to melee attacks.