Basically your looking at getting 2 wireless routers. The first one should be a commercial
grade device operating in the station mode to receive the signal from the shore based AP and the second one can be the identical unit operating as an access point or just a an office grade wireless router depending on whether you want to distribute to multiple computers
on your boat or cover a number of adjacent boats.
If you are just looking to distribute signal to multiple computers
onboard then you can install the office grade router below. If you want to rebroadcast to the anchorage you'll want another commercial
grade unit topsides.
If both units are operating topside then you need to keep the AP's frequency as far as possible from the frequency used by the shoreside AP and you need some seperation between the antennas both vertically and horizontally to avoid interference
IMO if you are going to redistribute a shoreside AP signal to the general public you need to get permission from the owner of the AP. They'll probably find out quickly anyway.