Bottle caps are needed for hyper training in Pokemon Sun/Moon. Hyper training allows you to increase a Pokemon stat’s IVs to 31 at level 100, making them extremely crucial for getting perfect IV legendary Pokemon in the games. This guide helps you get bottle caps easier and faster than hunting for shards in Poke Pelago, which could take months to get.
Repeat this method to farm bottle caps for hyper training. There is a 10% chance of getting a bottle cap for fishing in this area. Other items I obtained while fishing were pearls and sticky barbs. Avoid wild Pokemon encounters, obviously.