Star Coin 1 - As soon as you head underwater, there will be several rotating pipes on the left side that alternate currents. Swim between the pipes through the gap in the left to find the first Star ...

You can't miss the first Star Coin, though you'll need to jump on a Cheep Cheep to get it. Don't stand still too long while waiting for the Cheep Cheep, because the barrel you're on will sink into the ...