This idea began during [#076 - White Noise 2], but I thought I could do more with it than a footnote.
Again, this is a concept design around the iPhone / iPod Touch - specifically using the accelerometer and the touch screen to provide gameplay. The premise of this game is that you indirectly control a bunch of boats. You can do this via two mechanism. The first is wind, the direction of which you control based on the tilt of the device (if the Touch can actually tell you how tilty it is, perhaps you could even control the force of the winds by tilting it more or less).
The second way is by interacting with the sea by drawing currents on it. Just tapping the water will cause a ripple to go out from your finger. Draw a path with your finger to create a deliberate current. Draw little circles and you'll create a whirlpool (great for sinking ships). Draw a sine wave with your fingers and you'll create waves that will wash up against the shore. I've tried to show this in the image with the smear tool, but it didn't come out all that nicely.
Let me give you an example of how a level might play out. In this mission, there are three different missions happening (simultaneously or sequential depending on how easy it is to multitask). The first is that you must get a mail ship safely to four different ports, avoiding enemies. The second is that you must conquer an enemy base on the other side of the island. The third is a ship sent out to explore an uncharted area of the map.
The mail ship is a relatively easy one. The ship will automatically plot a course to the next port, going in a straight line. By giving it the wind, you can get it to move faster, or make the wind go against it to make it slower. You can draw currents in the water - these will not greatly affect the ship's course, but when the ship is near a port, it will throw out a mailbag into the water, and you need to use currents to deliver it safely. Perhaps there could be a fog on the water that you can use to hide the ship, moving it with the wind.
For the base conquering thing, a ship will automatically be sent out to attack the base. It will fire cannons at it automatically. Little guys will come out of the base to attack you as well. Strong wind will affect the trajectory of the cannon balls. The attackers might put mines in the water that you have to divert with currents (or create waves to send them back). One of the trees behind the fortress catches on fire - aim the wind towards the fort to have the fire spread. Draw whirlpools to swallow the rafts the enemy take to confront your ship.
For the uncharted island, the ship gets so far and then crashes into a reef. The crew members manage to get to lifeboats before the ship sinks. Without sails, the current is all that controls the destination of the lifeboats, so draw currents to steer the boats to the nearest islands. Adjust the wind to blow away fog covering potentially dangerous reefs. Draw currents to keep the sharks from getting to the boat. Calm rough currents by swiping in the opposite direction from them. When the lifeboats reach an island, the first mate will throw a message in a bottle into the water, and it is up to you to guide that bottle through the currents back to a nearby base where a rescue ship will be sent out.