Walkthrough: The second Island Trial, the one of Captain Lana, takes place almost unknowingly as you enter Brooklet Hill. Once you seamlessly enter the challenge, Lana will note of strange happenings in the area, and it's up to you to investigate. In the process of doing so, as you descend the hill, you will Battle multiple Wishwashi at Level 18 in their normal, weaker form.
Once you reach the bottom of the hill, you will have to swim out to sea to Battle the Totem Pokemon, which Captain Lana will give you the Poke Ride of Lapras to do so. The Totem Pokemon is another Wishwashi, but this Time at Level 20 in its School Form. When in its School Form - above Level 20 with more than 25% of its HP - Wishiwashi is one of the strongest Pokemon in the Game. Again, the Totem Pokemon will have its Defense raised by one stage stage, with the ability to again call for help, this time from other Wishwashi and Alomomola (all at Level 18).
After defeating the Totem Pokemon and all of the Pokemon in the Totem Battle, you will have completed the Trial. You will then obtain Waterium Z from Captain Lana.