N.B.: My comments apply to a fresh-water boat, so I don't know if they'll apply to you.
We used just plain old VC-17. That's that the PO used, and he raced the boat. (And never lost, apparently.) That's what all the other racers in our sail club used. So I figured it was good enough for us
We just painted-over last season's paint. VC-17 is semi-ablative, so you don't get a lot of build-up, doing that.
After the paint had been on a couple days, we lightly scraped the hull with these things that look a lot like large razor blades in holders. (You can get them at Home Depot, etc.), then burnished the hull with crumpled-up newspaper. The bottom ended-up smooth as a new-born baby's backside
Next spring we'll wipe her down with acetone, which will remove any paint remaining from previous seasons, lightly sand a couple small areas, here-and-there, then do the above. I also have a couple small bubbles in the keel that need looking-after.
Two extra gallons
?!?! For a 25' boat? We covered the entire hull of our 30' boat, with a bit left over, with two quarts