Hi people! Im very sorry that I almost forgot to post here what I did in the end! I finished my project some 4 months back. If you had followed all the posts you would be knowing that I had mentioned that I used a kinda propeller. Actually that idea got failed completely. Any underwater robot developers, dont use propeller because it just shifts the balance and the robot just sinks. If you are taking much care about the body's stability, which is the toughest part, then you can use propeller.
Ok here is what I did - This is my new own simple idea and with this I developed another small robot, smaller than the previous. It works like this:
* I had two 12V DC submersible suction motors, say A and B.
* I buit a small tank inside the robot.
* A is connected between outside pool and the inside tank of robot.
* B is connected between inside tank of robot and the outside pool.
* Actually the robot is positively buoyant, so it floats when there is no operation.
* Using A I sucked in the outside water into the tank, so the robot got sunk.
* Now I switched off A and switched on B to pump the water out of the tank, so the robot moved up to float on surface.
* I repeated this operation continously in pulses to make the robot oscillate about a point under water.
* when the 4 paddles(old idea) are rotated, any kind of motion can be achieved in sawtooth trajectory.
* Actually I had less time to make completely successful, yet I got 198 marks out 200.
Thank you all for the posts.