There is a rental fee, not sure what it is. We brought our own. I believe they need to be transparent. We purchased some online pretty cheaply and they were different colors and had no problem using them.
Others had non-resort ones as well a few weeks ago Publix had some for a real cheap (under$5) price but we passed and didn't really spend enough time at SAB to need one.
We stayed at BC last July. I think the tubes were about $10 per day to rent . Someone usually gave my kids theirs as they were leaving the pool area. We never actually had to rent one.