Hi, Bogino. If the underwater cameras you mentioned are the Kodak kind with the elastic rubber band-type strap you can put on your wrist, you should get some decent photos with them. I say "YOU" should get some decent photos. Your children, like most children, will probably take pictures of whatever fish they saw a moment ago, or the ocean floor, or whatever catches their fancy. Also, the cameras are a little complicated to work, in that you have to wind them after each shot. Most children that age will not remember to do that. And if you try to wind too hard, it could jam.
Also, your children will have enough to do just snorkeling. I take those disposable cameras along when I do shallow scuba diving and snorkeling, and I like the cameras, but I cannot imagine my 8-year-old trying to take any decent pictures while snorkeling. If I were you, I'd do this: Let your children each have a camera when they get into the water and are getting ready to snorkel. Let them take pictures of each other standing up in the ocean with masks on, etc. Normally you would not get to take a camera into the ocean like that. That should be fun for them. The cameras work fine in the open air. I often take one to the beach since I won't care if a wave splashes over it.
Then take the cameras back from them, and when you all start to snorkel, take pictures of your daughters doing it. Go underwater, and shoot them from there. It's a totally different perspective, and will be so fun for them to see the pictures later.
Use one of the cameras (any remaining exposures) in the pool, too! Back onboard the ship, let your daughters have the cameras back so they can use them in the pool. They'll take photos of their friends -- how fun! Better than fish! Also, it's fun to take a picture of someone jumping in, and water play in general.
Bottom line: The cameras are fine. The photos you get depend on the skill (and desire, opportunity) of the photographer. And I'd rather have photos of my children snorkeling than of the things they saw while doing it. But they'll love being given the cameras! And after they try using them for a short while, they would probably be happy to give them back to you (with plenty of unspent film!)
You are a great parent to be doing all this. The
Disney cruise....snorkeling....underwater cameras! They'll have a blast!
P.S. By the way, we did Snuba at Coki Beach. It's like scuba-diving, but you're limited to 20 feet, you don't have to be certified, you can do it as young as 8 and it's fun. I'm a diver and I took my 8-year-old to do it, and it was the highlight of his trip. I highly recommend it. Let me know if you are interested.
Have a fantastic trip!