A centralized server with thin clients would be preferable. For this option, the thin client could send a GPS reading to a central server and the server will respond back with the map data. I'm not sure if this solution would be too slow however. The screen wold need to be refreshed every few minutes with maps and markers for certain points of interest.
I believe this solution would be the easiest to maintain if eventually thousands of ships are using the application.
So I guess a ship would need a satellite internet connection to be able to send its GPS location and also be able to access the map application? I'm not an expert in these matters, but I would guess such an internet connection has pretty low bandwidth, so you would have to be careful about the quantity of data you're sending to the ship.
If this is just a collection of GPS positions, this shouldn't be a problem (even with thousands of ships). But you _will_ have problems if you're sending the map data (either in vector or bitmap form) to each ship. So I guess a Web map application is out of the question - you'd need to implement a thick-client mapping. Or at least pre-cache the Web map tiles on a local Web server on the ship, so it doesn't need to download the map from internet while at sea.
A lot of assumptions on my side, but there you go