If this is true, the short answer is: You got the wrong box.

No, if he's got a WRT54GS he can load the dd-wrt firmware on it and use it
as a client just fine. I know, that's exactly what I've got in my boat.

Once you have established connection to a remote WiFi, it is possible
to plug this into the WAN port on your WRT54GS and then have in effect
a WiFi 'repeater' on your boat. You can then have your laptop just
connect over YOUR WiFi network you have set up using the WRT54GS.

Yes, if you want to have both a link to shore and a wifi network wirelessly
on the boat you'll definitely need two boxes. One can't act as both. One
can "supposedly" use some wifi devices as bridges or repeaters but that cuts
the bandwidth in half (can't do both at once) and in my experience it's
generally unreliable.

I have used this setup a few times on Viking Star, but once it is all
set up and attached to a remote WiFi, I have not found a way to get
back into the Wireless Client (I am using a DWL2100AP) without
disconnecting the Ethernet cable from the WRT54GS and plugging it back
into the Laptop. One needs to do this in order to be able to select
which remote WiFi network to have the Wireless Client connect to.
However, once it is configured and working, it is really nice to have
a reliable WiFi based network on your own boat :-)

I do it all the time without disconnecting anything. The on-boat network is
always on the same wifi SSID; conveniently called "boat" on our vessel. So
the laptops aboard always stay connected to the same network. I just surf
to the shore-link device, also always on a fixed IP address as part of the
"boat" network, and use that to pick an on-shore SSID. Works well.

-Bill Kearney