I see it as a tile which you can punch out (you punch 4 boats out). I suppose that each player owns 1 personal boat of his color at the beginning of the game - he can use this boat as a secret "weapon" once per game (the boat is removed from the game after use). But I don't know - this is how I would use these boats.
maybe somebody can post the official rules with the mark "spoilers".
Actually that makes more sense than what I was thinking. Somehow I got the idea that it was a round card indicating nothing but tiny boats to add to the variation of rounds. But I should have realized that this would have been impossible -- there arent enough tiny boats in the game. So yes, they do look like punch out boats color coded to match different colors of stones. A little moe interesting...