What are buildings used for recreation and lounging, near marinas called? Something like a club house but only for private use. Generally owned by very wealthy people with yachts. The words coming to my mind are "lounge" and "outhouse", but I'm thinking of something on a grander scale.
In the UK the building that houses a yacht club is called the clubhouse, e.g.
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.