First head for the SV Dam, then once you are there go down the hill (don't go too far though).

Then head into the area where the trees are, there should be a hill there (if you are in the right area).

Once there follow the hill up and continue the path on. You should be on the other side of the water bridge.

Now go near the water gently and slow and hold both triggers at the same time to head in the water, then if done correctly you will be in the water.

Note: you can't do tricks in water or else you will bail!