If you are set on Windows this might help ...
Configuring Windows XP as a Network Bridge
The way I'd go about that is to get the laptop up on one interface and then alternately on the other interface before trying to bridge them, that way you have a better idea what is causing the problems if/when it doesn't work.
Edit - a really easy test of all of this would be to get a cross over cable and hook your laptop with the working wireless connection up to your desktop, then see if you can ping the host router and whether or not you can DHCP an address. Once that works then you can get it all working wirelessly on your boat.