Putting this into a ticket so as not to be forgotten. Don't know when I'll get to working on it again.
I've been wanting to add something to the underground lake in Semos Mine_n2_e2, and naturally thought about making some mermaid creatures to put there. Now the idea for an underwater kingdom like Atlantis has spun off from that.
There could be a quest item like a diver suit players could get from a sailor npc (someone on Athor ship maybe?) that would let them use certain portals to underwater maps. The npc could ask the player to go diving for sunken treasure and thus give the player a diving suit for this task.
Other possible underwater locations include: below Athor ship herself (for the quest mentioned above), and the Frogmen's pond at Ados Rock.