A waterproof computer is nonsense for most computer applications on a boat. Sure, you may want a waterproof (touch)screen outside, for gps/chart plotting purposes but all other pc usage (e.g. reception of RTTY weather forecasts or grib files, trip planning, Wifi/internet, electronic logbook) is done from the chart table.
Of course the computer should be fanless, so you don't suck salty air into it.
On my 34 ft sailing boat I have a (rather old) waterproof Simrad gps chart plotter with a tiny screen for outside work. The boat computer, a Sumicom S625F with a Pentium Mobile processor and a 2,5" HDD, is installed under the chart table. The screen is a comfortable standard 15", running directly on the battery. When the going gets rough I replace the standard usb keyboard with a flexible waterproof one, but the main input device is a Logitec trackball anyhow. The pc has it's own gps input from a gps-puck. To reduce battery charging I use rather severe power management rules.
I've been using this setup for 4 years now, both on the North Sea and the Med, never a moisture or overheating problem.