No port is easier to snorkel at than Grand Cayman. When you exit the pier you simply need to turn right and follow the road. You will pass the semi submersible sub headquarters and a bit beyond is the Paradise Bar and Grill. Here you can rent snorkel equpment if you did not bring your own for about $12USD per day. Because you can not anchor on the reefs there are buoys clearly marking the location of the reef. A diver need only go a few yards
further down the main road to
Eden Rock, a dive store that rents tanks and does escorted dives. Also not far from the pier, after exiting the pier turn left and follow the road. Within walking distance are cheeseburger reef out from the Burger King and and further down is Cemetary Reef, out from the Cemetary that you will find beside the main road.
There is a kayak rental business when you exit the pier and turn left, that does snuba tours as well.
In addition there are many locals advertising to give you an island tour, or just a shared van to Seven Mile Beach.
So many possibilities, that its hard to choose the right one. I guess thats just one more good reason to comeback again.