OK, so now we are being told the new box we have ordered will not work
with the Linksys WRT54GS router we already have.

What 'new box'?

It's pretty simple, you need two radios. One to make the link to shore and
the other to provide an on-boat wireless network. I've had success using a
pair of WRT54GS routers to do this. I've loaded the dd-wrt firmware on
them. It works quite well. There are any number of other ways you could
configure it. But given the low cost and wide availabiilty of dd-wrt
compatible routers, well, why bother?