This reinforces the danger I realized I was in when my wife and I were snorkeling in some rough waters during an excursion. The water was pretty rough and I started to feel a little sea sick on our way out to the snorkeling spot. We got in the water and things got a little better (because we were not on a boat pitching anymore). How ever, after a while, I started to feel a little sick because the sea was still a little rough. Even though I am a strong swimmer, I realized that I could quickly get into serious trouble if I started to vomit while snorkeling, so we cut or snorkalling short and headed back to the boat.
Later on that same trip, we tried snorkelling off of Serinity Bay at CC, but this time the water was so rough that I realized that it was too easy to lose each other in the waves. One of us might get in trouble and the other wouldn't even know it.
But it is because of stories like this why even if you are a part of a tour group, you should still follow the buddy system when snorkelling.