When I restored my '68 Starcraft, I applied Herculiner to the inside of the hull. It's a riveted boat, so after replacing a dozen leaky rivets, I used that to ensure no leaks. 11 seasons later, and not a drop of water in the boat.
When I used to post on the aluminum boat repair boards, it was common to do this as well as the outer hull. Everyone who did it, loved it.
Incidentally, I already planned to do the outer hull of my Starcraft (I am watching a few eBay auctions for gallons of Herculiner now

), probably late May when I get time. It's due for bottom paint anyway...
As for slowing the boat down, I wouldn't think it would much that you would notice. Ever see the Mythbuster's episode where they did a Ford compact up like a golfball with dimples and it actually went faster ??
I think it's a good idea if you have holes and pits.