Never use acetone on polyethylene, it can deteriorate or dissolve it.
Stick with detergents. If dishwasher detergent is not strong enough, try an industrial detergent. You can find these at Home Depot/Lowes
If you own a pressure washer, that can help remove stubborn stains. Use a wide nozzle, never a narrow one.
You can also try a detergent containing Clorox such as
Clorox Cleaner and Bleach. Let it soak in for a while, then rinse with plenty of water.
Also, try using a
Magic Eraser, but go easy as these are mildly abrasive.
For a list of Hobie dealers in your area look
here. There has to be some in Northern NJ/NYC.