You can use the same GPS like a Garmin GPS60 on land or sea. The big thing is what features you want in a GPS unit which decides the price. You could use a Hema or VMS portable unit as long the maps for boating are excepted by the software. You could ask a company like Johnny Appleseed Your Garmin GPS and TomTom GPS shop for GPS Australia on GPS unit that supports both offroad, onroad and boating maps.
bmurray has some good advice, I'd say something that is splash/water proof and you can download map outlines to. Then sort out which ones have marine charts and land maps. Probably stick to Garmin or Magellan. Johnny Appleseed will set you straight.
From personal experience, i'd avoid Magellan, purely because they're very stingy with maps... ie, you'll have to go and buy maps, where with Garmin you can download them. However, before making a decision, I thoroughly recommend checking out: GPS Australia .:GPS Australia:.
i thoroughly recommend the Garmin 60 CSX ... it does a brilliant job weather its in my balloon or offroad hooked to my lappy its tough and has been bounced around and still workes well ... and im pretty sure they can adapt offshore maps easily from what i read about it when i read up on it before i bought it!
have fun reading up about GPS theres heaps out there!!!