I second talk2Dug. I also live aboard full time, have 2 laptops and 2 desktop. The oldest is around 4 years old and is a standard micro-atx with standard desktop parts (except for an industrial 12v power supply). So far no issues related to moisture. I do keep these machines in the cabin but am working on a touch screen in the cockpit for charting with a mini-itx AMD Geode based pc below.

Oh, let me add. We took the 4 year old computer offshore via Bermuda to the Caribbean. So I could say we were not inland with it. I also know a few offshore sailors who swear by pc's aboard.