I'd get a portable USB/Firewire interface for the laptop, then just use Garageband or Logic Express to provide backtracks. For the USB/Firewire interface try M-Audio. You should be able to get one for pretty cheap. That way you can just plug in your electric guitar or mic, and record straight to the computer in a fairly small package. You didn't say if you were recording acoustic or electric, but that should take care of both. If its just acoustic, you can skip the USB/Firewire interface and just get one of those new USB mics (try Samson). I think they are around $70 and offer high quality sound. Garageband will provide all the backtracks you could need, maybe not for pro use, but i think it should get you started off reasonably.
The problem is this; if your on a sailboat, the sea water tends to make anything metal/sometimes electronic rust or just stop working. Plus if you get it wet, your pretty screwed. If the boat is big, I wouldn't worry about it, but if its a smaller boat and you have a fear of getting stuff wet, opt for an all in one recorder. This is hard to get backtracks for, but it beats killing your laptop.