Closest useful method to go underwater for extended amounts of time would be to find the equivalent of a bathosphere, very early 1900's primitive ball-shaped submarine diving bells. Although beyond normal medieval technology, with dwarven and gnomish tinkering and engineering and maybe "Steampunk", it seems like it wouldn't be all that unusual.
It takes both an expert GM and an advanced player to properly play an underwater RPG if there is no magic involved. If there is magic involved then specifics may not matter.