There are some IPX7 rated cases for the Fujitsu series of tablet PCs. One interesting way to interface a laptop with the GPS is to use a NMEA/Bluetooth Multiplexer.
That would allow you to use the notebook/tablet PC almost anywhere on the boat, and still get real-time navigation data.
For the Mac, the software I'd recommend is MacENC and for the PC, I'd recommend Cap'n 8.
If you wanted to use the Tablet to remotely control a Notebook or Desktop PC below decks, you can use VNC as a protocol, and use WiFi to connect the two machines.
The HP machines, tablet or not, have always had some serious quality control issues IMO. The Fujitsu machines seem to be pretty reliable, as do the Panasonic Toughbooks. There are also some other brands of "semi-rugged" or "fully-rugged" Notebooks, but they're not really "name-brand" manufacturers.
Being a Geek for a living, I'd be happy to answer any specific questions.