Walkthrough: Taking up on Kahuna Hapu's advice, you will need to hitch a ride to Exeggutor Island from the Chief of Seafolk Village. However, you will have to travel to the island by yourself. When you arrive, the weather will be fierce and the Exeggutors found around the island will be restless. It will be up to you to help them, and after you do, you will gain a path to the flute you are seeking.
Island Scan Pokemon:
Chesnaught
Pinsir (Desturbing Exeggutors)
Wild Pokemon:
Initial Pokemon (Levels 42-45): Tropius(20%), Exeggcute(20%), Pelipper(20%), Exeggutor(20%), Gastrodon(10%), Pinsir(10%)
Weather Battles (Levels 40-43): Castform, Sliggoo
Items:
Sun Flute - From Pedestal
Big Pearl (Hidden) - Small Alcove
PP Up (Hidden) - Behind Tauros Rocks
Revive (Hidden) - Path Behind Exeggutor