You gotta be careful about small auto or airplane power sources for laptops, because they often are not suited for small sailboats. This is because they are meant to work at voltages normally present when a car engine
is running, i.e., 13 to 14.5 volts or so. Often, below about 12.8 volts, they don't put out enough voltage to charge your laptop battery
Take, for example, the typical DC power adapter for IBM laptops. These work fine at higher input voltages, but on a sailboat where battery voltages are often below 12.6, they won't charge your battery. They will run the internal laptop battery down. I have two such adapters from IBM, and they both exhibit the same behavior.
Solution: an adapter which works well at lower input voltages. Best I've found is from Lind Electronics
(they make robust adapters for IBM and many others), and they have models which will work with input voltages as low as 10V or so. I've used one happily for several years now. Auto Power Adapters, Auto Air Adapters and Power Packs for Laptop and Notebook Computers.