Patience is when you know the thing is coming. I've never heard of such feature to be discussed. I really hope they will, but I agree with Lorska that most probbably it's not on the radar. I just wanted to toss my opinion on the matter.

I do understand the implications of any new feature to be added. How is going to be used? "Exploited" (NPC's don't dive, player can dive each time he's in danger and the AI will generally be confused...)?

Double the stamina usage underwater. Ask for special gear to do it (diving). Prevent useless features by eliminating the cause - all items to have buoyancy. Do what ever you want just give the players the things that they fight for, don't let the loot to drop in inaccessible places ........................