gasoline is a lot more flamable than diesel, most boats use diesel and most fuel tanks are below the water line.... metal doesn't burn...
it might work against a jet aircraft but it's totally impractical against a boat. unless the boat is made of wood or fiberglass.
if anything, it seemed that boat was in greater danger of being swamped then of being lasered.
P.S. The title is totally misleading... it should have read "Navy uses powerful and megaexpensive laser to ignite the gasoline in a small unprotected outboard engine while leaving the little bitty boat intact."