While they do not require paint for any protection per se, they will allow biological growth to adhere to them. Since painting the bottom will invariably get paint on them anyway, might as well make the job complete and paint them over. Transducers may need a coating as well to maintain function, depending on the type of bottom paint applied.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.