yes applying sealant to a car is always a benefit, it protects the paint from contaminants,UV damage etc.
fk1000p hi temp sealant is very good, jetseal109 is again very good, both worth considering, jetseal being the easier one of the two to apply and remove

for your boat look into the Gtechniq range

or the jetseal or fk1000p are both very good for boats.
ive got a rotory so dont take much notice of the DA machines but the g220 and the das 6 are basically the same but i think the DAS6 is the newer model your just paying more because g220 has megs stickers on it(someone correct me if im wrong on this)