With Sharpedo under your belt, you may be reminded of those areas back in Melemele Island surrounded by sea rocks. Whenever you've got a spare moment, call Charizard and glide to Route 1 - Hau'oli Outskirts. You'll arrive just outside Ten Carat Hill.
Smash through the nearby rocks using Tauros and make your way towards the shore below Professor Kukui's lab. Summon Sharpedo by the edge of shore, then jet towards the big island in the south-east corner. You'll need to smash through the rocks along the east or wide side of the island.
Past the rocks, land on the south-eastern shore. In the middle of the island is a lonely TM80 Rock Slide. Were you expecting to find more here? Afterwards, return to the sea and jet towards the south-west corner of this area, then exit to the west along the south side of Ten Carat Hill.
In the next area, continue west. Towards the south-west side of Ten Carat Hill, smash through the sea rocks to reach the sea cave. Beyond, you'll emerge in a watery path inside Ten Carat Hill. Continue ahead. Waiting at the end of the short path is TM64 Explosion.
Boom! That's all you're going to find. Leave the subterranean cave and head towards your next destination, wherever that may be.