If you want to use it a lot on board. I suggest you find a direct 12 volt adapter and avoid using an inverter plus an A/C converter to power the laptop. My boat has plenty of 12 volt sockets and I found a converter on the Web.
You''ll use less power overall. Going 12 Volt batter to 12 volt computer is more efficent than making 120 volt power then making DC power again. Sames goes for cell phone chargers. The one for your car is the way to go unless you want a real A/C adapter when plugged in at the dock.
The new article today by Don Casey on inverters is a good explination of why this is and how much it matters.