Swimming/Snorkeling. Pretty much everyone is going to jump in the water and go for a self tour of the shoreline or reef and etc. As a certified boat captain and diver, make sure who's in the water on such tour, all of them know who's in the water, where they are and always do a head count from time to time. It's always a good idea for at least one person to tether a PFD while swimming. It can be on a longer line so it won't interfere with that persons swimming. The PFD should be those stupid looking bright red kind found on every boat that know body wants to wear. This PFD will serve two really good functions. 1). If someone in this self guided tour gets tired they can rest on this PFD . This is especially good if they can't touch bottom. 2). This bright red PFD will be much easier to spot then some half summered swimmer by those dinghy racers who are trying to get their dinghy to skim off the water at Mach 1 speed. Those dinghy racers are NOT looking for swimmers, but rather where their hat just blew off.