I am uneducated in this particular field but my understanding is that salt water is extremely corrosive. Having reliable energy is not just about installation, but also about maintenance and continued operation. The latter is a challenge in a corrosive salt water environment.

In comparison to keeping the panels clean and cooled in the middle of a desert?

Serious answer? Yes. Brooms and mounting panels a foot off the ground works fine. Heat reduces efficiently but by a very, very small amount. Not long ago there was a company posted to HN which puts panels basically flat on the ground, and has a little robot run up and down sweeping them off in Texas.

Much simpler and cheaper and less error prone than a floating array of panels exposed to salt water and waves.

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.

Would panels be even efficient for such a project though? I think concentrated solar is better in this case, as it generates power at night, more stable during weather changes, and not subject to heating issues.

The solar panels would shade out all the photosynthetic plankton below, no? Nothing large-scale on Earth is zero impact.

Salt water is rough on infrastructure, and far less convenient to access.

Floating windmills sounds like a fun concept. I never heard of it but in theory, with cables securing it to the bottom and weights to keep it upright, I could see it work.

There are existing floating turbines already.

https://en.wikipedia.org/wiki/Floating_wind_turbine

Scotland was big, at one point, on using sea-power. Their idea was more about using the rising/falling/tidal energy to generate electricity via motion:

https://en.wikipedia.org/wiki/Pelamis_Wave_Energy_Converter

There was an article a while back about placing solar panels on California aqueducts. This wasn’t really worth the cost in the analysis, until you considered preventing water loss from evaporation and saving on canal cleaning. I expect solar power from the ocean would lose money, if floating it on land is barely profitable.