Am I maybe making this more complicated than I need to?
If I allow my PC to connect to the WiFi, note what the subnet is and then simply set up a different subnet for use on the boat I should be OK shouldn't I? If I turn everything off on the router (Wireless, 3G, etc) and just use it as a switch for the LAN then everything should talk OK.
The WiFi gives my PC a 192.168.1.x address. So I can set up my lan to use, say, 192.168.13.xxx. The printer should 'just work' as all it has to do is communicate with other devices on the LAN. I can tell the VOIP box that its default route is my PC (the 192.168.13.x address) and it should then be able to see the outside world shouldn't it?