This area used to be known as Secluded Shore and pretty much nobody on Ula'ula Island visited it. But in this timeline, it's the third Mantine Surf spot and a pretty lively place, all things considered. From the entrance, head east to find the scoreboard. To the south is the Mantine if you want to try some surfing.
Even More Mantine Surfing
As a reminder, each Mantine Surf spot has its own course, and the one from here is much longer and trickier than the previous two. Also don't forget about the incentive for trying every course. If you can top the scores of all 4 surf spots, there's a Pikachu gift waiting at the Heahea City Surf Association!
Anyway, call Stoutland and search the sand to the east of the scoreboard. Just below the rocky outcrop, you can find a buried Pearl . Further east, past the scoreboard, is what looks like a Slowpoke paradise. Head on over to the cluster of Slowpokes by the shoreline. To their right is a shiny item: another letter in a bottle!
When you have time, fly back to Malie City, then head for the north-east corner of the Outer Cape. Speak to the Rising Star standing by the palm trees and he'll request a battle.
Rising Star Nainoa
Delibird is a pushover. It has a double weakness to Rock, and is weak to Fire, Electric and Steel. Mudbray is still unevolved so shouldn't bother your team, but be cautious of its powerful Ground-type High Horsepower.
As thanks for delivering the message and accepting his battle request, you'll receive a very valuable Pearl String. Afterwards, you can fly straight back to Ula'ula Beach. Back at the Slowpoke area, search around the north-east corner, to the left of the Slowpokes. You can find a buried Big Pearl that's pretty valuable as well.
Of course, feel free to dig through the berry stack near the back. Once you're done, return towards the entrance to the beach. Just below the entrance, on the left, are three surfboards placed away from the others. Search near the middle of the surfboards to find a Dive Ball that one of the surfers must have forgotten.
Afterwards, have Stoutland sniff around directly south. Between the rightmost stall and the sea, there's a buried Pearl that you should chuck with all the others. Continue sniffing around, this time towards the west. Between the leftmost stall and the sea, there's a juicy Big Pearl that's sure to weigh your bag down.
Finally carry on west, behind the stalls to find a Revive that hasn't sunken into the sand. Before leaving, check out the BP shops if you need to, especially if you've earn a lot of BP by excelling in Mantine Surf, from stuffing your Pokémon Bank boxes, etc. Afterwards, return to Route 12 and exit to Route 13 via the west.
BP Move Tutor
The BP shop on the left can teach the following moves: Iron Head, Aqua Tail, Pain Split, Tailwind, Thunder Punch, Endeavor, Focus Punch, Icy Wind, Zen Headbutt, Seed Bomb, Laser Focus, Trick, Drill Run, Magic Coat, Magic Room, Ice Punch and Wonder Room.
Many of these moves are useful for competitive play, but for the main story, you should prioritize the attack moves, like the elemental punches.