Ok, so a Mastercraft is not a toilet, but I tried this for the first time and it's so easy it feels like cheating!
Couple of bottles of The Works toilet bowl cleaner in a garden sprayer, and an open hose in the other. Just spray it down, and rinse it off 10 seconds later; perfectly clean!
I already tried purple power, simple green, and a few other things, all with lots of scrubbing and little effect.
Shouldn't really have to do this again, as I won't be leaving it in the water (unlike the previous owner), so it should stay pretty clean after a good wax job.
I made sure to thouroughly rinse down the bunks, trailer, and cement driveway with plenty of water.
I did make the mistake of getting a bit on the rudder, so for now I've got a half pink rudder (looks like a penny after sitting in vinegar). Hopefully it'll oxidise again so it looks better.