Hi Choochella, Sorry for the late reply I just read your post. Hope it is not too late to do you any good.
Antigua, Long bay beach is great for snorkeling. This is actually, my favorite beach. The sand is the finest anywhere. The reef is not far off the shore.
St. Lucia, I would recommend the island tour but go with a local taxi driver. Just get off the ship and find a local taxi that will do the same tour as the ships itinerary. You will see more do more and have more time to do it for less than half the cost.
Martinique, If you do not speak French you may not find this island as unique. They do have taxi drivers that speak English. They can take you on an island tour and you can see all of the highlights of the island including the small town that was buried in volcanic ash many years ago. If you speak French you might want to take the ferry to the small island just off the coast that the locals all go to. Warning it is a topless beach. You can stop at one of the local bakery shops and get some bread and some wine to have a French style lunch. This is also a good island to buy your fine French perfume.
Grenada, I would recommend doing the island tour with a local taxi driver. He can take you to the nutmeg factories and show you all of the island. This is really a very pretty island lots of trees and spices
As far as taking two watches and extra special precautions ---. We always just plan on being at the ship at least one hour before departure. Everyone on the island knows how important it is to get you back. Everyone will remind you and do everything to get you back in plenty of time. But you are the one responsible to get back. So if you are not comfortable - get back even earlier or just take the ships tours.
Hope this helps.