In any case, he will need to get two boxes, right?

Yes, two boxes. One to make the link to the shore and the other to provide
an on-boat wireless network. You could get away with just one making the
shore connection. But then you'd have to use a wired connection to any
other computers onboard. Which you could do, as I've done, by running a
single ethernet wire down from the shore-link router to a wired switch.
Then connect your computer(s) there. I found it was better to have wireless
to avoid the tripping hazards cables pose.

Do you give your shore-link / wireless client device an fixed IP
address within your 'boats' IP range?

Yes. On the boat I use a 172.16.x.x/255.255.0.0 range of addresses. This
helps avoid conflicts with anything on shore using a
192.168.x.x/255.255.255.0 range. The access point on the boat uses
172.16.x.2 and the shore-link router in the radar arch uses 172.16.x.1. So
I just keep a web browser bookmark setup for http://172.16.x.1 and use that
to configure the router when we get anchored. It's worked really well.

And when I mention 172.16.x.x the 'x' is a number from 1 to 255. I simply x
out the ones I use for the sake of, well, privacy. That and should someone
else happen to get networked along with me somewhere on the Chesapeake it
cuts down on the likely conflict. You could just as well use 172.16.1.x,
172.16.88.x or anything else up to 255.

172.16.x.x using a Class B subnet of 255.255.0.0 is one of the private
network numbers that can be used for devices that are not directly connected
to the Internet. The 192.168.x.x Class C 255.255.255.0 is another one.
(note the difference in subnet masks) You can use either. Most folks don't
know about private addressing so get stuck with just the 192.168.x.x
numbering. But know this, you SHOULD NOT just make up numbering. Always
use the legitimate private network numbers. Otherwise you'd be screwed
if/when you actually need to route to something on the same addressing as
something you've 'made up'.

Are you able to continue to communicate
with your shore-link no matter what the shore base station uses for
its IP ranges?

Yes, routing is such that a local network is going to get priority anyway.
But since I use the 172.16.x.x range of private addresses the chances of
conflict are really quite low. Even if the shore network was on the same
range it's my on-boat network that's handling the packets first. I have the
shore-link router acting as a client but also running DHCP services. The
on-boat router runs nothing, it's just an access point. The ethernet wire
goes between the LAN side of each router. The WAN ports on both of them are
unused. I actually run my gear all into one ethernet switch. That's also
where my Raymarine E-80 is networked along with the SR1000 Sirius marine
weather unit. It's all worked quite nicely this past summer.

-Bill Kearney