WiFi repeater
I think what you might want is a WiFi repeater. This will just repeat a shore-based WiFi network into your boat. Most such solutions have a hi-gain antenna for the external side, as the signal you're dealing with is likely weak.
I've found that some of the Linksys router/repeaters have wall-wart AC adapters that feed 12V DC to the units, so you can just cut the transformer off the cord and wire it to your panel. In this case you'd find a hi-gain antenna for the unit (which are available), but making it into a repeater (and not just an access point) requires some hack-ery.
A purpose-made, dual-radio, hi-gain repeater is over $200. Power might be more of a challenge. Some use 48VDC (which is common in telephone networks), but you can get a 12 to 48 V DC-DC converter for about $50. Check Hawking Technology for repeaters and L-Com.com for power conversion.
I had considered this, and if you have free WiFi at your marina it's great. But as I've cruised around I found WiFi in harbors, and it wasn't cheap - like, $10 for a day pass! I never found any WiFi provided by my cell carrier, which would have been free. I've opted for a cell carrier who is rolling out 4G LTE infrastructure and already has good coverage where I go, plus free or cheap WiFi mobile network sharing via my Android phone. I can connect up to 5 laptops to my phone at once, and when I have 4G coverage, from 1 to almost 20Mbps.