This part is optional, especially if you have a portable lawn sprinkler that can cover the surface of the slide pretty evenly. Mine couldn't, so I had to give my slide a nice, even spray down before I could use it.
Pro tip: To level up your DIY slip 'n' slide experience, assign someone to aim the hose at every slider! I found it to be much more exciting than having just the portable sprinkler on.
Image Source: POPSUGAR Photogray / Taylon Faltas