The Gunboats are not a fleet of small vessels shooting at people from the water. Which would actually be pretty nice, but I guess it's a bit too similar to the Sentry.
Anyhow, those Gunboats are special boots for the Soldier. They are pretty heavy (since they are reinforced with depleted uranium I would assume) and thus the Soldier can not wear these and wield a secondary weapon at the same time. Makes no sense, really, but I don't make the rules. That's straight from TF2.
So, what do these boots do? They lower the damage done to the player when rocket jumping by sixty per cent. Sounds neat.