Mom3kids, on the Golden Eagle, you are not out in open waters. The catamaran parks a teeny bit offshore. IMO, you do not have to be a great swimmer. You may swim off the catamaran and stay and play in the water - or you may snorkle.
We could jump right off the catamaran into the crystal clear water too. If you do not wish to jump in the water, the crew guides you off the catamaran and you ease yourself into the water off a ramp/ladder/staircase type thing (from what I remember). You can ease into the water on one of the many noodles they supply, so it's not like you have to swim to keep yourself up. It feels very safe and comfortable. The waters were very calm.
As for the kids, they would have to be comfortable getting into water that is over their head as they need to make their way (a short distance) to the beach, while floating around with or without a noodle. The catamaran is always in sight and close to the shore. We felt safe at all times.
It was so relaxing to float around in the water and this is just a really fun excursion. We swam most of the time and actually didn't make our way on sand until our beach time was almost over.
Snorkeling is optional. The nice part about the Golden Eagle is, if you are not snorkeling, there is no time "wasted" waiting for people to finish snorkeling (I'm not trying to be mean here, I just didn't know any other way to word this). Once you reach the beautiful Flat Island, you have reached your destination with no other stops
Great island music all the way there and back - sandwiches and drinks included. The sandwiches are similar to mini-subs, so are more of a snack than a full lunch. Unless you eat several of them, you might still be hungry enough for lunch on the ship.
On the St John Five Star, the boat parks a bit more off-shore. You can either snorkel off the boat (which is not in a scary open area) or you can make your way to the beach on a dinghy. Again, very safe atmosphere.
On both excursions, you are dealing with a fantastic crew.
Maybe someone can help you with the time frame on St John as far as catching another excursion. Hope this helps.
Tink