It might interest you to know that prior to 1955 (I think that was the year) Ford also used positive grounds for their cars.
The reason for using negative grounds in automobiles is that it reduces the fouling of spark plugs slightly. Other than that it really doesn't matter which way the juice flows (in a design sense - because it will determine which way the electric motors turn).
In a plastic boat it doesn't make any difference at all which one you remove first. The only reason you want to remove the negative side first on a car is that if the wrench accidently touches somepart of the body while you are removing the negative it won't spark - and once the negative is off the same is true when then removing the positive side. With a plastic boat you wouldn't get the short if the wrench touched to boat itself so it really doesn't matter a bit. Actually it doesn't make any difference when working on a car either, if you pay any attention at all to what you are doing.