The Mayhem Motorboat is not a named location in Fortnite, so you won't see it on the map. It is a landmark, so there is no actual marker or waypoint to lead you to it. You can find the Mayhem Motorboat next to the big shit that leads to the Hydro 16 site, right next to Lazy Lake. We have marked the exact location on the map below.
The Mayhem Motorboat is an area that can take on challenges, and it's a solid place to land at the start of a turn. Unsurprisingly, there are plenty of motorboats in the area, as well as a Choppa on the island in the middle of Lazy Lake. The area is also filled with ammo boxes, loot boxes, and even loot sharks. Fishing is plentiful here, so you can also stock up on weapons and supplies this way if you wish.
You can also run time trials here which is one of the challenges of Season 3 of Week 4. To do this you just need to get into a boat and then follow the start line of the against the watch. A timer will begin to count down, and when it reaches zero, you will be able to move forward, through the rings that will appear. Follow the rings around, making sure to cross over any new ones that appear as you cross them. There will be additional rings on the course, we assume that in an attempt to confuse you, be sure to aim for the most recent ring each time. Eventually you'll be pulled back to the start line, and the time trial will end when you cross it.
Once you're done in the area, you can choose to head to Lazy Lake, Misty Meadows, or hop into the Choppa and go anywhere on the map, which makes it a pretty good starting point to do any match.
