When snorkeling or swimming, if you hear a boat you need to pop your head out of the water and wave and do your best to ensure that the boat captain sees you. He really can't see you when your head is in the water. Of course, if you are not in a designated snorkeling area that has mooring buoys (such as Hol Chan) then you should be towing a snorkeling buoy and have "snorkel cover." Snorkel cover is having your boat captain monitoring your progress and it is his job to make sure other boats are aware of your presence. If your captain is also your snorkel guide then definitely one of you needs to tow a buoy to make you more visible to boats. Remember the glare of the sun, the waves and floating debris can make you less visible to boat traffic. Although a boat captain must keep a keen eye looking for swimmers, the swimmer must take steps to ensure safety too.