I have been looking at getting an HF rig for my boat also, and I have found that the amateurs tend to prefer the amateur rigs and likewise for the marine users for the marine rigs. I have an amateur radio license, but I am currently leaning towards the marine rigs. I found that the marine rigs tend to be smaller (like the amateur portable models), but are focused on having memories and features for the marine user who likes to just have access to particular channels. Amateurs like to surf the bands, so their radios have more features for doing that. Most newer marine radios can sync with your GPS for DSC and other marine features like e-mail, lower power consumption, ease of stowing, and top on my list, water resistance (though some HAM rigs have this too).
As others mentioned you have to have licenses for using either of the HF band plans (marine and amateur). Getting access to an amateur (who likes to tinker with antennas) to help with your antenna to check the SWR and that you have a good ground plane would be beneficial, since on boats that is one of the major problems for getting good propagation (and every boat is different).