High Seas Combat
The foundry hop - This can be used to move around the map, all without building a shipyard. What you do is build a foundry (or other water building) in one of the rivers. Postion it so that the top left corner is on another island. When your peasant finishes (or you cancel it) he will be on the other island. Start up a new town here, then move on to the next. You will still need a transport to move your fighting units to enemy islands.
You can also get to their island, throw up a barracks and send in a few attackers before he notices. You may even kill all his peasants. :)
Sapper through the rocks to get other islands too, but make sure there is no one there if you are unprepared. If he doesn't notice, you could wipe him out with this. This is a great trick if he has no land forces. Just land near him, sapper through, and Presto! You've just wiped out his town. His ships won't do much good without peasants to back them up and make more.