Make Cannon Boats not flammable by giving them the #minecraft:non_flammable_wood tag or just removing whatever behaviour makes them flammable in the first place

This makes sense as Cannons aren't flammable, so it's weird that you could stick a Cannon Boat in a furnace and somehow burn it.

Boatload makes their Furnace Boats non-flammable as well, following this logic because Furnaces aren't flammable.

Well true.