Heat reduces power of PV Panels by up to 10%. Looking at a random spec sheet it's -0.35%/C so from 45C to 75C the difference is roughly 10%. I would not call it "very very small". Also increase in temperature reduces the lifetime of the panels.

That said I too think that taking that hit will never be offset by all the faff of dealing with ocean water, waves etc.

Moreover the big installations in Morocco have not been PV but concentrated mirrors in which case any increase in heat is improvement and brooms are more important.

Brooms cause abrasions, especially when they are moving sand along glass or composite plastics. Non lubricated wipers are fine (but not ideal) for dirt / dust, but not at all comparable for sand.

solar panels have been being swept with brooms for decades, it works fine

Yeah, and as far as I am aware, they are all kept clean with water. I'm not aware of a single installation that isn't hooked up to water facilities, let alone plonked down in the middle of a desert.

I'm really not sure why you think washing them with water is vital to make a solar installation worthwhile. Panels have been getting / are dirt cheap, efficiencies have been going up, and that trend will continue for a long time. If $ cost to wash is more than $ increase from cleaner panels, we won't wash them.