Karen, welcome aboard! You are going to just adore your trip on the Disney Magic. I sailed on the Magic last month with my own four-year old son who had a blast! One of his favorite activities was swimming in the Mickey pool!
He is a decent swimmer, but certainly could use a life jacket for deeper water. However, this time we didn't bring it because we've found he really prefers to swim in
Mickey's Pool. Now, when I say "swim", it's more like wading up to his waist and splashing around with newly found friends. The Mickey pool is only about two feet deep, so it's the perfect spot for the younger cruisers to have a blast while Mom and Dad sit on the side to watch. Your little man probably wouldn't want to bother with water wings in the Mickey pool. He might also like to take a few solo trips down Mickey's slide, just above the Mickey pool. That was my little guy's favorite thing to do at the pool. Up, then slide down, go up, then slide down... he could have done that all day and night!
Goofy's Pool onboard is a family pool where your little guy might like to swim with you. This pool goes up to four feet deep. There he could use his life jacket or water wings, and you are certainly welcome to bring your own onboard.
I bet you are really looking forward to this very magical cruise, Karen! Please let me know if you have any further questions before you set sail with Captain Mickey and friends!