This task is similar to that of the New World mission, where you are required to search and find the letter F in a loading screen.
To successfully complete this mission, first you must have access to the loading screen. To activate it in the game, you must finish at least 8 Tasks / objectives of the open water mission. You can then put it on the loading screen of your game, and watch if you find the letter O!
Boat Locations Guide: Fortnite Chapter 2 Season 1