Walkthrough: As you arrive at Brooklet Hill, you will meet Captain Lana. This is where your next Island Trial will take place, the one set by Lana herself. During your Trial, you will descend through Brooklet Hill as you herd the Wishiwashi down into the bottom area. Each Wishiwashi can be found in the splashing water - accessed with the Lapras Poke Ride that Lana will give you - forcing you to battle them after you find them. However, sometimes the splashing water will uncover Dewpider instead. You will have to battle through these as well until you find all the Wishiwashi. At the bottom of Brooklet Hill will be where you battle the Totem Pokemon.
As you reach the bottom of Brooklet Hill, it will begin to rain rapidly as a group of Wishiwashi begin to form into their School Form in the center of the lake. However, as you surf over to them, a large shadow will appear before the Wishiwashi can become this form. The shadow will burst out from the ocean, revealing itself as an Araquanid, the same that is the Totem Pokemon for this Trial. This Araquanid will be at Level 20, having its speed increase by one level as the battle begins. It will also call upon help from Masquerain and Dewpider (all at Level 18). Defeat the Totem Pokemon and all of its SOS Partners to complete the Trial and obtain Waterium Z.