Here's a source for you that mentions that even some typical formulations of stainless steel are susceptible to corrosion in saltwater:
https://www.clintonaluminum.com/304-stainless-steel-in-seawa...
https://www.clintonaluminum.com/304-stainless-steel-in-seawa...
No one is arguing that it's impossible to build a bear with solar panels. The point is that you can't slap a cheap panel from Alibaba on a float and have any assurance that it will work reliably. Thus costs go up and the whole idea may become infeasible.