The water has sea grass in is, close to shore. This make for some nice snorkeling, since little fish and critters can swim around it in very shallow water. There isn't a "drop off" that I've found yet. And, the lack of wave action makes it great for snorkling beginners. You can walk along the beach and find areas that don't have the sea grass, if it creeps you out, but I love it. I have seen sand dollars, little purple fish, little yellow ones, sea cucumbers, crabs, and lots more, all in the shallow areas with sea grass. If he was to get panic'd, all he'd have to do is stand up, and he'd be in waist deep or less. You have picked the right place, if you are asking if San Pedro is the right place. I'm not familiar with Isla, and have never stayed at AquaMarina, but I hear they are great. Hope you have a great time.
Live so that when you arise in the A.M, Satan shudders & says..
'Oh sh t..she's awake!'