In order to unlock this you need to clean the Deck steps first. Do keep in mind you need to be on the mission "Clean the Fishing Boat" and not "Clean the Frolic Boat".
Deck steps being:
Bow
Main Deck
Steps (0/2)
Press esc or start and scroll the list and select them to make them flash. This should make it easier for you to find the correct parts. These are all Wood. So using Soap Nozzle + Wood Cleaner may speed things up alot but keep you surroundings in mind. If you want to play safe just aim carefully with water only. The main deck has a very big coverage and you almost need like everything. So spray around the other items that you dont need with care.
You can replay this level by going to Free play -> Locations -> Clean the Fishing boat