Right now for IASW it’s just 1 test boat so I don’t think it has anything to do with current holiday crowds. They are probably starting to ramp up capacity on as many attractions as they can for the future when they expand park capacity and start re-opening more resorts.
I honestly don’t know whether the science behind it is that much worse. The plexiglass prevents a direct spray of spit particles coming directly from behind or in front of you so it probably does something. It certainly has a psychological impact. I’ve seen schools using the face shields and plexiglass for teachers too. I still think it’s probably better than just loading all the rows with no plexiglass.