To destroy structures with a Motorboat or Motorboat Missiles in Fortnite Chapter 3 Season 1, you first need to find a Motorboat. The best place to find a Motorboat in Fortnite Chapter 3 Season 1 is in the lagoon on the east side of the map. There is a particularly high concentration of Motorboats south of the Mighty Monument. There are Motorboats scattered around the coastline and in some of the lakes and rivers too. The map below shows all the Motorboat locations in Fortnite Chapter 3 Season 1. Some of the locations marked here have more than one Motorboat.
When you find a Motorboat, the first thing to remember is that the controls are different from land vehicles. The button that functions as an accelerator in a car, shoots missiles in a Motorboat. The movement controls for a boat are actually pretty similar to those you use on foot, only you have more inertia.
The definition of a “structure” in the quest to destroy structures with a Motorboat is pretty loose. Basically, anything man-made and destructible counts as a structure, so that includes small objects like boxes and crates. You’re best off destroying stuff using the missiles because most structures are on land, and if you attempt to destroy them by ramming them with the Motorboat, you’ll probably run yourself aground.