Walkthrough: Just off Tapu Village is Route 14, the path that leads you the Abandoned Thrifty MegaMart where your next Island Trial waits. There are plenty of Wild Water Pokemon and Water Trainers found down this coastline.
Wild Pokemon:
Surfing (Levels 28-31): Finneon(40%), Tentacool(40%), Pelipper(20%)
Fishing (Levels 10-31): Magikarp(79%), Wishiwashi(20%), Bruxish(1%)
SOS Pokemon #1 (Levels 10-31): Gyarados, Wishiwashi, Bruxish
SOS Pokemon #2 (Levels 10-31): Magikarp, Wishiwashi, Bruxish
Fishing Bubble Spots (Levels 10-31): Magikarp(50%), Wishiwashi(30%), Bruxish(20%)
SOS Pokemon #1 (Levels 10-31): Gyarados, Wishiwashi, Bruxish
Zygarde Cells: There are two Zygarde Cells found here.
Trainers:
Swimmer Keoni - Level 31 Pyukumuku
Fisherman Hisato - Level 30 Feebas, Level 31 Gyarados
Collector Kawika - Level 30 Togedemaru, Level 31 Gabite
Items:
TM30 Shadowball - Middle Ruins Water
Max Potion - Near Route Entrance
Pearl - Near Tau Village Exit (Hidden)
Big Pearl - West of Second Ramp (Hidden)