There's a blue coin in water to left, another in water to right. You have to chase it. There's a third in the water. A fourth is from the swing - just get onto the swing and use your jets to get you swinging!

This level is about red coins. There's a red coin tucked in the reef further to the right. 2 of them swim, and you have to chase them. The other six are just in nooks. It's pretty easy if you just swim around and look.

Super Mario Sunshine Walkthrough