About halfway through this short route is some water (it’s so short that you’re not going to miss it). Use **** Surf and head out onto it. Head north-west across the water to a little alcove to find a Water Stone . Head back onto the water and head east. Continue to the very end where there are some steps.
Move onto the land and up the steps. Head north, then west along the edge of the cliff to the item at the end. Pick up the Nest Ball , then backtrack to the stairs. This time head south. Near the end is a hidden Expert Belt . Use your Dowsing MCHN to find it.