One container of suitable size (wheely bin is ideal) 3boxes of caustic soda crystals from B&Q. Fill bin to level of boat with warm water, add the crystals, stand clear and wait for the thermonuclear reaction to subside! Then add boat. Check every30 minutes, the paint will soon be off, once clean power wash or thoroughly rinse with clean water and sponge.
This has the added benefit of also cleaning the wheely bin, pour the liquid slowly into outside drains or Gulley's to also remove fat/dirt built up. Don't pour the last 6 inches away, this has paint residue decant this into plastic 1litre milk bottles and take to local council recycle centre.
Don't worry about it going down the drain, its sold as drain cleaner!