Depends. Where will you be using them? On the ship, your youngest can only go in the splash pad area. (someone correct me if I am wrong. Last time we cruised, non-tty trained kids had to stay out of the pools.). So, he wouldn't need it on the ship. Your dd can swim in the Mickey and Goofy pools. Mickey pool is quite shallow. Goofy is fairly deep. Maybe 4 or 4 1/2 feet deep? If she is not comfortable in that deep water, you may want them.
If you want them for an excursion, I would check with the excursion people. I bet they will have floatation devices available.
Perfect, this is the info I was looking to hear! Ok...I'll pack only her items!