What do you put under a fiberglass tub when it's installed on plywood?

That depends on what the manufacturer recommends/requires. There are products that require bedding of some sort, many options depending on specific tub, and there are products that require absolutely no bedding at all, just set the tub, level, anchor/support it by manufacture's guidelines and you are good to go. If the manufacturer does not require bedding, or does not give specifics on the bedding material, you can ask 20 different professionals and many times get 20 different opinions. Emphasis on OPINIONS. My suggestion would be to tell us the exact make and model of tub, or simply contact the manufacturer to get the best info for your exact tub installment. Good luck and please do let us know what you find.