Instructions
Go past the halfway flag and swim up to the surface; you'll see the first Star Coin, above a red pipe shooting a jet of current at you. You can't swim to the coin, as the current is too strong. Freeze the large fish swimming around near the surface. When the resulting ice block floats up to the surface, jump onto it, then across to the Star Coin.
Freeze another fish or two beneath the twin vertical red pipes shooting current down at you. The second Star Coin is between these two pipes. Get on top of the ice block, and ride it as it floats up to the surface, against the current. The block will break as soon as it touches one of the pipes, so be ready to jump off and get the coin before that happens. If you don't get it on the first try, swim off screen and come back, and there should be more fish for you to freeze.
Look to the right after you get the second coin and you'll see the third, near the bottom of the screen. Another pipe is shooting current, upward this time, blocking your way to where the third Star Coin is. Freeze the urchin floating back and forth here when he's exactly over the pipe. He'll hit the ceiling and drop back down, blocking the stream of current and allowing you to swim past and grab the Star Coin.