Underwater combat rules

The official word on GURPS 4e underwater combat rules as that there are none:

Currently, there are no unifed rules for this in 4e. The 3e rules were bad . playtesters who had experience with scuba, including a guy trained in hand-to-hand combat by the Marines, warned us off reusing those in 4e. So I wouldn't use 'em. In 4e, I suppose it will be its own PDF someday. Until that day, apply the extra movement-point costs on p. B387 and the penalties for bad footing on pp. B547-549. Drag is a complicated question, as it depends on cross-sectional shape, so it's difficult to generalize about its effects on melee damage . . . really, each weapon will need its own new rating.

Rules for GURPS Azeroth underwater combat will therefore be decided by GM fiat.  Most likely there will be magical and technological aids that will allow above-ground weapons to function underwater - for example, a Shoot Underwater type enchant for bows and crossbows, specially modified firearms and amphibious booster packs to allow steam armor to function underwater, etc.