Maybe I'm old fashioned, but we just put on liquid dish soap (no water) and let it dry on the hull(s). Then, when we arrived at the site, we washed the boat (or just dropped it in the water). The grime (even behind an F-250 diesel back in the day when you could follow the smoke trail to find 'em) washed right off.

Oh yeah, that works great with camping cookware when you're using a wood stove (on the exterior only). Keeps them from getting black.

If you are just going to drop the boat in the water rather than wash it, I would humbly suggest NOT soaping up the deck (makes for some tricky sailing when a soaped up deck gets wet!)