I know how to swim but you have a life jacket on the whole time you snorkel. When I went I wanted to go under water but the life jacket stopped me from doing so. I think you would be fine. Just relax and enjoy it.
This will be you, but with a life jacket of course!!!
I have done a lot of snorkelling, and my friends that don't swim did not really enjoy it. Of course, if you did it off a beach, where you can stand, it might be better, but they were really anxious out in the ocean, life jacket or not.
My husband can not swim, and sometimes snorkeling for him is ok and sometime it is not. When in calm water you can stand up in it is no problem, just stick your mask in the water. If the water was deep or choppy or both he had a hard time and couldn't do it, even with a life vest. If you are stopping in the Cayman Islands on your cruise I would recommend swimming with the stingrays tour. They drop you off on a sandbar that is waist to chest deep. My husband was very comfortable snorkling there and had a great time. We also tried snorkeling off the beach in cozumel and it was too rough for him. Hope this helps!
it all depends how comfortable you are in the water. If you are comfortable floating and sticking your face in the water then you should do just fine. You will have on a floatation device and it is much easier to float in salt water.
With a vest and fins swimming is really a non issue . The vest takes care of the floating and the fins take care of the "swimming" . Do it with a buddy who can assist if you become aprehensive at any time in the water.
My wife is barely a swimmer, confident in the water but not much more. We have snorkelled as many as five times in five ports and do it as often as we can. With a vest and fins, anyone is a snorkeller. If my wife gets tired or has a problem making progress against a current, then we hold hands and she gets a bit of an assist from me. Sometimes she gets out of the water earlier than I do but she insists on snorkelling . The world beneath the waves is quite facinating.
Just my two cents worth.
I used a novel method of taking a "non-swimmer" out snorkeling. We were in Hawaii and I used my son's bodyboard. I had the Non-swimmer simply lay on the board with his face over the side with the snorkel in his mouth. I then used my fin power to pull the board around by the attached wrist leash. He could see clearly and breathe naturally without worrying about swimming. On a cruise ship, a bodyboard might offer some problems stroring the board and traveling with it but perhaps a blowup device might offer the same. I agree with each poster that a vest will be helpful but I must admit that most "non-swimmers" have a definate fear of just being in the water. The floating device worked well the allay those fears. Anyway, have fun and enjoy whatever you try.