Your best bet might be to use a laptop as the basis. They are already designed with power efficiency in mind, and you won't need an external screen and keyboard for local problem solving.
I would also consider having a raspberry pi 3 or similar as a companion. Services that must be up all the time run on the pi (e.g. network admin). The main computer only gets kicked out of sleep mode when required. The pi 3 needs less power than the newer pis, while still having enough computing power to not lag unless pushed hard.
I definitely agree with SSDs. HDDs don't do well when rotated when running. Boats are less than a stable platform.