We had been keeping a dinghy in our slip for the last 3 months until our mates came back from NY to retrieve it. We agreed to watch over it while their boat was being hauled out on land and worked on.
Algae, seaweed, tube worms, and barnacles all attach to the bottom and become one big growth that is just nasty.
We pulled it out of the water and this is what the bottom looked like.
Warning! Graphic Images: Don't let your bottom get this dirty! :)
Next we sprayed it with a high powered water hose
Tools needed were a bottle of '409' and two hard bristle scrub brushes
After spraying the '409' and letting it soak in a bit , scrub it around in a circular motion
As you can see, it is already getting cleaner
Spray off the remaining areas and wipe clean.
close up views of the dirty bottom.
close up views of the "worms" .
close up view of "barnacle". 
