It is for the teen club - and their key to the world card is coded to only allow those of that age into the proper kids club.
and it is not a pool or hot tub - when I asked they called them splash tubs - basically the size of a hot tub, but no heat, jets but not big enough to swim in either - just a place to hang out in the water.
on longer cruises they let the Tween group "invade" the teen club but I do not think (not for sure) that they do that on short cruises as we have never been on a short cruise.