What are buildings used for recreation and lounging near marinas called? Something like a club house but only for private use

In the UK the building that houses a yacht club is called the clubhouse, e.g.

Brighton Marina Yacht Club Clubhouse

Note that in the UK, outhouse usually means something like this https://upload.wikimedia.org/wikipedia/commons/b/b9/Amish_Outhouse.jpg

I don't think such a thing exists at a marina. If it were a private beach or dock on private land, that would be different.

But as the author, you get to make up whatever you like.

cabana a small cabin or tentlike structure for use as a bathhouse, especially on a beach or by a swimming pool -- Random House.