Thank you both labatt and sailingdog for your replies.
I suppose my main area of confusion lies in the difference/function between a bridge and a router; especially with regards to my particular application.
On say a standard wireless router at home for example it creates a wireless link between a "cabled" modem and the users computer. Could the home modem itself be considered the "bridge" in this instance? Is this essentially what a home modem is?
Since there is already a wireless internet signal being broadcast by the marina ISP would a standard wireless router not only transmit between both itself and the ISP as well as itself and my computer thereby creating a usable internet connection link?
Would my first scenario with regards to the USB antenna, a notebook and my current Apple router not work with the 2 devices "bridged" in the Win XP networking configuration?
Again, this is to achieve a wireless network within my boat and while only at dock with no more than 400' between my vessel and the ISP signal on shore.
I'm essentially trying to make this as uncomplicated as I can while using my existing equipment.