During the Cayo Perico island scope out, you can do an optional side mission that will weaken the guards during the Cayo Perico Heist Finale. This optional mission consists of finding a randomly placed cutting powder and contaminating the water tower with it.
The cutting powder can be found in one of the three locations, and it spawns randomly. There are two water towers on the island, and you can contaminate either one of these. You do not have to put the powder in both of them.
Cutting Powder Locations
There are three possible locations where you can find the cutting powder.
Cutting Powder Location 1
The first location is in the middle of the island in a shed where some workers are manufacturing drugs. The box of cutting powder is orange in color and easy to spot as it glows brightly. The box will be next to a trash can.
Cutting Powder Location 2
The second location is to the very west near the Main Dock area. You will find the box under the shade of the longest building in the area. You can also find some guard clothing here.
Cutting Powder Location 3
The third and final possible location is near the weed field in the eastern part of the island. You will find a small shed area with some materials in there. The box will be on the ground. You can also find a grappling hook here on the cardboard boxes.
Water Tower Locations
As mentioned before, there are two locations where you will find the water towers. You can contaminate any one of these, and the guards will be weakened during the Cayo Perico heist finale.
Water Tower 1 - North Dock
The tower is at the back of the buildings near the North Dock of the island.
Water Tower 2 - Main Dock
This tower is in the Main Dock area.
Once you've contaminated the water, guards will take longer to detect you, and they will always have a bad aim. Their shooting accuracy will also drop. We all know how the GTA Online NPCs have the "perfect" aim. In other words, aimbot. Doing this mission will help you bring that "perfect" NPC aim down a notch. This is particularly useful for those who are going to attempt this heist aggressively.