I didn’t say it was a deal breaker, only that it was worse than working on land, eg a solar farm in a field. Working at heights is also a risk, and working at heights above water is even greater than that. As for the significance of working at heights on land vs working on or near water, really a hypothetical comparison there is moot - at that point, you should be considering the specific circumstances for your risk assessment.

At the end of the day, it’s unusual and a greater risk than other work. So people are more likely to charge more for it. However the bigger costs will probably be whatever specialist equipment will need to be developed to clean the panels quickly, as well as mitigations for situations where live cables fall into the water, or even just the equipment you use to clean falling in and needing to be repaired or replaced. Again, none of this is insurmountable, but it very apparently puts off risk-averse developers (and/or the venture capitalists that finance them). If it didn’t, they’d have already done it by now.