My idea of boat% was a little different. Something that I've done (for a fun personal challenge) but never thought of doing as a category is for a race with the boat 'starting'/ending on the couch EP, maybe I'll suggest it properly at some point with more articulate wording:

Complete all EP's except for eclipse, starting the boat as early as possible without re-summoning it and ending on couch EP. This requires all EP's (except couch) that involve the boat to be performed before commencing the boat race (buoy, three on shipwreck, long dock, both tutorial castles, orange container, swamp dock, mountainside to bunker and desert). You'll also need to solve the crate in town as well as a number of treehouse panels before commencing the race (just so you can get into treehouse once the boat is in motion). I did it until the three-way split on the way to the backdoor drawbridge of the treehouse, the remainder of which are easily snipable from the ground/the backdoor itself.

After you've done the prep, you can summon the boat at any dock, set it to any speed (other than stopped) and then must complete the rest of the EP's and must never resummon the boat again, ending only on couch EP. You may increase/decrease the speed (other than stopped) while the boat is in motion, but may not alter the course.