From Adam, a level with plenty of water in - and this room, plus some small floating islands. More details? Well, there isn't really much more to say. Rooms to swim in, with openings into the next room down at the bottom - it can get a little confusing which way is which, so remember to come up for air occasionally...
The little islands have either the shield, megapower or doubler, depending on which room you are in; it is all pretty simple, but ought to be quite fun for a game with a couple of other people. After all, the NAB maps don't make enough use of water and swimming, perhaps... I would prefer it is the map had more of an "open air" feel to it, with a large skybox, and maybe some grassy surrounds, but it is a small download, and an agreeable place to hang out in for a while.