Hide behind the small pillar on the island to hide from the Colossi's attacks. When he stops, jump in the water and swim round towards his back and climb up. Go towards his head, and you will see 3 blue teeth like objects. Hitting these with your side directs him either left, straight or right. Aim for a bigger island, when you can jump onto it.
Now you are higher, once again hide behind the pillar so his attacks don't hit you. When the Colossus comes to your level and lifts up, you can see the weak spot on his belly. Make the jump and attack this part with your sword.
You will have to do this a couple of times, as when he goes into the water you need to start the process again.