A Boat Repair Kit can be obtained early in the game by destroying the large piece of Debris at the player's spawn with a Hammer.
Upon use, it recovers 100 regardless of what boat it is used on. It can be used 3 times before being completely consumed. The Boat Repair Kit can be used while on the boat by right clicking on the Kit. It can also be used without getting on to the boat by dragging it on to the boat from dry land.
The Boat Repair kit is also present in the Hamlet DLC.
The ability to craft Boat Repair Kit in the Hamlet DLC was introduced in the June 7, 2019 update.