This will show you how to build a robot boat from cheap and easy to use materials, I think it is also a good example of reusing water bottles.

I used two water bottles, polymorph plastic, two motors and propellers, tape, a box, a battery and a motoruino (Arduino clone designed to work with motors, servos and sensors).

Step 1: Materials Needed:

Two water bottles
Two simple dc motors
Two propellers
Isolating tape
Box
Motoruino (or any other microcontroller and a h-bridge)
Battery

Step 2: Add the Motor and the Propeller to Each Bottle

Take out the bottle cap and insert the motor on its mouth (you can use some insulating tape if you see that there is space between the motor and the plastic).
Use polymorph to seal the bottle. If this is the first time you're using this material you may find these tutorials helpful:
Attach the propeller

Step 3: Connect the Two Bottles

I used a polymorph extruded tube to pass give structure and pass the motors' wires and insulating tape just to support the box.

Step 4: Make the Electrical Connections

If you use a motoruino, you just have to connect the battery and motors' wires to the board. If you use a normal arduino you will have to use a shield or mount the h-bridge on a breadboard.

I used a SFR05 sonar, if you use it too, check this example: http://www.robot-electronics.co.uk/htm/arduino_examples.htm#SRF05 Ultrasonic Ranger

Step 5: Have Fun!

Put the battery and the arduino at the box and mount it on the robot.
Upload the code to the microcontroller. If you want to have a look at my code, download it, it is attached.
Have fun!