Your deck or dock should be cleaned and brightened regularly to keep stains, algae and fungus from getting a good grip. Then it should be sealed with a quality wood sealer to prevent water damage. Pressure washing your deck is not as easy as you might think. Too little pressure and your deck cleaning will not effectively remove all mold, mildew or dark stains. If the pressure is too high, the surface of the wood could be destroyed. Hiring a professional to do this for you is your best bet.