Only the third point is actually "really" relevant, the thing is though if he swims up he may be able to type /map show... Also who said that he isn't already on the visible on the map!
OK, logical thing to do is swim up, and then log out when you begin to take damage. For the first few seconds after logging in, you don't take drowning damage.
what i suggest: build a pillar over the water. build down into the water until you have a 5x5 dirt house ( or something like that ) lock a chest with your items in it. get cords, write them down and drown yourself. you can always go back and they will be safe. ( reason for dirt house is so its not noticeable on the livemap