Or can dig an air pocket if close to shore or bottom. Or play a scenario that have modified weapons. Or you can modify item config file yourself. BTW wrong section to ask in, should be "General".
I must admit, I have pistols, pulse rifles and sentries modified for underwater, for eg: { +Item Id: 20, Name: PistolT2, Ref: Pistol ... { Child 0 ... AllowAt: "UnderWater, Planet, Space" ... } ... } I did the sentries because I often build by and/or over water's edge and often had problems with Talon base attacks spawning in the water where nothing could get at them (bad design / bug IMHO).