This is a situation where the referee has to use his/her best judgment and experience.  Is there a valid medical reason for the tape?  If so, will the tape give the swimmer an advantage.  Taping a swimmer's finger to cover a cut would normally be acceptable, but taping all of the swimmers fingers together, and in effect forming a web, would not be acceptable, as it would clearly give the swimmer an advantage, no matter what the medical reason.  Also, kinesio tape is never permitted.  If you go to the USA-S website and to the Officials FAQs, Question #8 provides guidance issued by the Rules and Regulations Committee.