"the expected life span of a laptop is 4 years on land."
According to who and for what reason? I know some corporate types who bash 'em up every 18 months and need them replaced. And some kids who got laptops for college, treated 'em right, and are still using them in grad school, more than 4 years later, with zero damage. I got a good seven years out of my last one and then passed it on.
"therefore it will run less on a boat" Or, therefore it will last twice as long on a boat, where it isn't obsoleted because it doesn't need a new OS or new software or more horsepower to run the same old nav software and email.