Be sure to take the boat up the river. As soon as you make the first turn, look to the wall to your left to spot a small waterfall and a hole. Use the boat to jump into the hole. Once inside, follow the small path and walk into the small waterfall to finally reach the secret.
From the last secret, go to the other bank of the river and look to your right. Behind a tree and between the woods is a small opening covered in green. Walk through it to get the secret.
Continue going up the river and stop when you reach the "Fresh Scallops" sign. Here, dive down and look for a crack somewhat hidden by some plants. Blow it up and navigate through the hole to emerge at the secret area.