Hydro 16 in Fortnite is a landmark, not a named location, so you won’t see it on the map. You can find it to the west of the dam, a large building squatting at the rive side between Motorboat Mayhem and Slurpy Swamp.
There are plenty of loot spots here, and it is actually a pretty good place to drop in at the start of the map. You can search the building for materials and loot, then make your way to the island in Lazy Lake and grab a helicopter, pick up a boat at Motorboat Mayhem, or just jump in one of the many cars that spawn nearby.
If you really need extra loot, you are close to Slurpy Swamp, Rickety Rig, and Misty Meadows, so have plenty of options for great locations to loot up. The main reason to visit Hydro 16 in Chapter 2 Season 3 is for the Week 9 challenge to collect 200 metal there.
This is incredibly easy to do, as the vast majority of Hydro 16 is build from metal. There are large metal transformers outside, and lots more inside as well. The entire roof is also made of metal, so if you land up there from the Battle Bus, you can get the challenge finished up extremely quickly. After that, you can head into the rest of the map to finish up the rest of the Week 9 Chapter 2 Season 3 challenges that you can find below: