Eventually, you should report back to Aether House on Route 15. As you approach, it becomes clear that your assistance will be required.
Team Skull Admin Plumeria
We last saw Team Skull's big sister near the end of Akala Island. Her partners haven't changed, but they're higher Level now.
Golbat shouldn't be a surprise, although you should never underestimate one of these bloodsuckers. It's weak to Electric, Rock, Ice and Psychic-types.
Salazzle is the evolved form of Salandit. It's stronger, but still exceptionally weak to Ground-types. If you've got none of those, Water, Rock and Psychic-types are also super-effective.
Although you manage to drive away Team Skull, it appears the damage has already been done. If you want to rectify the situation, you'll need to head to Team Skull's headquarters in Po Town, alone. This would be dangerous for anyone else, but not you--a 11-year old or something child!
Before you head out, you'll receive a Rare Candy as encouragement. Now you really can't afford to fail! Exit Aether House and make your way west towards the shore on Route 15. A little ways to the north is a strange man in a white kimono; go up and speak to him.
Well, if it isn't Grimsley, one of the Elite Four from the Unova region! But what's with the kimono? Not that we're questioning his fashion sense; it just seems a little random is all... Anyway, the Dark-type user will aid you by adding Sharpedo Jet to your Ride Pager.
Needless to say, you can use Sharpedo Jet to smash the rocks in the water, just like Tauros Charge on land. Without further ado, head to the edge of the shore and call Sharpedo. From here, jet south from the shore, smashing through any rocks in the way. Woohoo!
Although Sharpedo Jet appears to make Lapras Paddle completely redundant, a side effect of Sharpedo is that all bubbling fishing spots disappear when you get near. Although it's no surprise that the rare fish are all afraid of the shark...
Past the line of rocks originally obstructing your path, you'll be by the south side. Follow the large island in the middle around, then land on its western shore. North from here is a short path with tall grass along the west side and a trainer walking along the other side.
Ace Trainer Karla
A decent mix of Pokemon. Alolan Vulpix is Ice-type unlike its normal, Fire-type counterpart. Therefore it's weak to Fire, Rock, Steel and Fighting-types.
Haunter you should've met at the abandoned supermarket. It's weak to Ghost, Dark and Psychic-type moves. Ground-type moves would be super-effective normally, but Haunter has Levitate.
Finally, Wigglytuff is the final evolution of Igglybuff. It retains the same Type combination, so smack it with Poison or Steel-type moves if possible.
At the end of the path is a big empty square area. No idea what this is supposed to be... is it at all related to the empty lots located in each of the Alolan towns? But this one is way too developed. Anyway, in the middle of this empty area is TM93 Wild Charge.
Pick up the TM, then head back to the water. From the shore, jet all the way north, between the tiny islands. Before passing the large island, you'll come across a trainer swimming between the land. If you want, you can travel along the far west side instead to avoid him.
The evolved form of Tentacool. Like its earlier form, it's weak to Electric, Psychic and Ground-types. The evolved version has decent Special Defence, so use physical moves if you can.
Beyond here, jet north-east. At the far corner, there's a shore where you can disembark. Towards the middle of this shore are a pair of trainers doing push ups. If you want to disturb them, just run in front while they're facing forward or speak to them.
Swimmers Yumi and Jake
Slowbro is one of the evolved forms of Slowpoke. It's weak to Grass, Electric, Ghost, Dark and Bug-types so you're spoiled for chance. It has pretty beefy Defence, so hit it with special moves if you can.
Meanwhile Golduck is the evolved form of Psyduck and another pretend Psychic-type like its predecessor. Take it out with the usual Grass or Electric-types.
North from the swimmer pair is a Dive Ball sitting comfortably in the corner. That's this tiny shore covered; hop back into the water and jet west from here. Along the north is the shore leading to the next route. Before landing, say "Alola" to the swimmer halfway along.
All pink Water-types and none of them particularly threatening. Sink them with strong Electric or Grass-types; especially Grass-types for Corsola.
Hop onto the northern shore and give Sharpedo a well-earned rest. To the far east is a fisherman with a Starmie. Search the base of the cliff north from him to find a hidden Pearl. After that, run towards the far west. Search the far corner to discover a buried Normal Gem.
To proceed, head up the slope a short distance east from here to reach Route 16. After visiting the Pokemon Center here, you can glide back to Tapu Village and take on the boss trainer--the trial guide--near the entrance of Route 15.
Trial Guide Katrina
Klefki has Prankster, which allows it to go first if it uses status moves. It's annoying for sure, but has next to no offensive preference. Smash it with Fire or Ground-types before it causes too much mayhem.
Skarmory has good Defense, so focus on special moves, especially Fire or Electric-type ones. It is adorned with Steelium Z, so Types that resist Steel (which includes the aforementioned Types) are also good to use.
Prove your worth and you'll earn a super-rare PP Max. This functions like 3 lots of PP Up at once, instantly boosting a moves PP value to its maximum. Of course, it's best used on a move that hasn't had any PP Ups.
In the waters of Route 15, there's... absolutely nothing new to be found. So just relax as you speed through the waters on top of Sharpedo.