Where the treasure hunter quest started, at the river, there is a pillar with a green and a red box at the bottom. Run up that pillar and cross to the other side of the river to climb up to the head of the snake.
Now cross the water by jumping onto the water lilys, enter the cave and go left. There is a hovering skull. Use Spinjitzu or a Ranged Weapon to move it 6 times.
Get out of the water. Where the red mushrooms are, go up to the left, where the big gate is. There is a small pond with a crossover to a picnic site. The brick is in this pond
Go back to the Main Water Area, where the red mushrooms are. Left of the mushrooms there is a passage with a cracked wall. Use Earth Spinjitzu, go through and do the Race.
Go back to where you came up the restored ruins and follow the river left (not through the tunnel). There you can cross over to another structure. Climb it. On top you can build a radio beacon.