But have you looked at the Apple Airport Express? It won't share a cellular modem but it will do all the other things listed quite well, for repeating and sharing a marina wifi connection.

If you want to share a cellular modem take a look at CradlePoint. They're the best at that, IMHO.

I endorse these. In both cases you'll need to use an inverter for 120vac or engineer your own 12v adapter.