To some extent it depends on how the marina does their security. A router will allow all devices to appear to come from a single address. The problem you have is you need the WAN port on the router to be wifi. There used to be some routers that could do that but they are extremely uncommon.
So you need to build your own. What you need is a extra radio to connect to the wan port. This device is called a client-bridge. It appears to be a end user machine. Many so called "extenders" or repeaters can run in client-bridge mode. It might be easier to just buy a actual bridge. You can get outdoor bridge from companies like ubiquiti. In your case it might actually help your signal to use outdoor direction bridge...