Super Mario Sunshine Tips

Secret Gelato Beach Coins
I'm pretty sure you could do this any time during the game (but I found it in the second level):

Find the sail-boat off to the left of where you arrived on the beach. Stand on the right of the boat, facing the trees/hills. Then start spraying the sand in front of you with water; soon blue coin will appear!!

Also, when you spray those weird duck things walking around, they will turn up-side-down and if you jump on them, either a regular coin or a blue coin will appear, depending on the color of the duck you sprayed!