A few points.
1. Most laptops have a 12V power adaptor available as an accessory (extra cost). Buy it.
2. Buy a second battery for when the first runs out. Not cheap but very useful.
3. Ensure batteries are Lithium Ion or you will have very limited battery time. Also the N MH batteries die much earlier.
4. Get a laptop with very good connectivity. Should have at the very least 1 serial port, 1 USB port and 1 PCMCIA slot. In addition you want a built in Network card and a built in modem for when you stay in areas that offer high speed internet or telephone access.
5. Buy your Nav software before you decide on a Laptop. Then get a laptop that meets the requirement of your software. ALL COMPUTERS SHOULD BE BOUGHT THIS WAY. This also includes determining which Operating System the computer uses. Most of the software is Old and works best with older versions of Windows.
6. Get a DVD reader and a CDRW in the laptop. This will give you the freedom to watch movies as well as to back up your data.
7. Check the warranties and reviews for the laptops. The marine environment is not friendly with all the salt, moisture and banging around. Suggest a touch pad over the "accu-point" for a mouse pointer.
Hope this helps