Are you running a sailboat or motorboat?.

For a sailboat, power efficiency is important. Typical laptop brick is
60 Watt, which is more than 5 Amps.
Current (no pun) laptops are too power hungry for most sailboats and
overspecified for charting. Avoid charting or any software that uses
too much processing power (MaxSea charting software that consumes
80-100%CPU), as more CPU is more watts from power supply).
A three to five year old laptop, Pentium iii, of 500-1000Mhz, running
Windows 2000 is ideal in minimal configuration. Minimal is no media
player, no email, no internet, no virus scanners, no office software,
no external network connections, completely stand-alone--dedicated to
onboard tasks only.
At this price point you can afford a spare PC, similarly configured for
the inevitable
failure. Salt water and damp are not electronic friendly and hard disks
do not like the motion on board.
If not an old laptop, you could do a Google search for "Car computer".
In summary the boat PC should be dedidated to on board activities only.
For email,web DVD etc etc a seperate modern unit could be carried, but
internet cafes are preferable and cheaper.
Finally a DC-DC converter (car adapter) should be used rather than the
laptop's power brick, which will be a source of electrical noise and of
power loss in conversion.