The ToughBook 19 and 30 will provide you the most protection if the laptop will be out of the cabin. Protection in the form of corrosion and oxidation resistance of the motherboard. Sea air is brutal on PC components, especially laptops (somebody open theirs up to view this oxidation on the copper components if you're feeling adventurous).
These 2 models are really the only 2 that have a moisture barrier, sort of like a sealed gasket. But, even that isn't totally waterproof. The 30 has much better daylight "readability". Both are GPS capable. Both have good shockability. I've dropped both from waist height and both kept on tickin'. AND It WILL be forgotten to be put away and will slide off your nav table despite it's non-skid coating--absolutely count on that.
Find some to play with in your area to see for yourself the difference. Have a knowlegeable salesperson open one up along with any other brand to see the differences in the inside. That is why they are more expensive.
Expect to pay $3,500.00 on up and worth every penny.