I'm just going to throw out some random ideas:
Labs: very friendly and can run loose, but at a Marina, you risk a dog who wants to be in the water. NOT a good idea, IMO.
GSD mix - whatever the dog resembles as another breed - that's what it becomes. Australian Shepherds also come in black/tan colors; Collies are sable; Cattle Dogs bring pricked ears to the equation.
Personally, I'd go with your first choice, either a working or herding-type dog. They are the most likely to stick around and not get into trouble; and generally are sociable with people and other animals (assuming the marina has folks who take their dogs on their boats?)