I have the same question as you have only I thought the kayak was on Barbados. Two years ago we did the kayak thing on St. Thomas and it was a hoot. The kayaks are unsinkable, floaters for two persons. We paddled out across the mangrove area to a secluded beach where we snorkled and had some refreshments (no booze ... no way to get you back if you drink too much!) I am almost 62 and my wife is 60. She walks 6 miles a day. I do the sam some days although not as regularly. Unless there is a lot of open ocean paddling you shouldn't have any trouble. Be sure to drink a lot of water as it's easy to get dehydrated. Also wear loose fitting long sleeved shirt and pants. The sun can be brutal when you sit in one of these with your legs stretched out.
We signed up for the morning kayak and afternoon snorkle on Barbados. Will report how it goes.