I figured out the clam thing. There are 13 that you can find on the visible beach area by the lighthouse and the sand castle beach. 2 at the light house - 11 at the main beach area.
By the way My son thought they should have a 3D sand castle building solo game. Like Rollercoaster tycoon.
He said they could call it SandCastle Tycoon. It would be fun. That little side game is one of my favorites in DODI.
now for the last 2 clams......
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There are some rocks you can get on. If you go forward you will crack yourself all up. But you can jump to other spots to a hidden clamming spot.