Here is the info I found, you can "borrow" life vests from Disney or bring your own...
Permitted at resorts:
Walt Disney World approved flotation devices may be used but they must be securely attached to the body and supervising adults must remain within arms reach of the person that is using the device. Lifejackets are available at the Resorts.
Any US Coast Guard Type II or Type III personal flotation device
Arm Bands
Swim Rings (split rings and full rings)
Beach Balls
Permitted at Walt Disney World Water Parks
Plastic Goggles that do not cover the nose
Swimmers aid devices that attach to the body. (Parents must stay within arms length of the child.)
Coast Guard personal flotation devices (PFD), water wings and flotation sewn into the bathing suit are permitted in shallow water. (Shallow water is defined as chest deep on the person responsible for the child wearing the swimmer's aid; generally 4 feet of water or less.)
Life jackets provided by the water park
Life Jackets are available for adults and children at all Walt Disney World Resorts and Water Parks.
Resorts:
Complimentary to resort guests, however there is a $47.93 fee if not returned.
Water Parks:
Complimentary, but must leave a form of personal identification such as
Room Key
Car Keys
Insurance Card
Credit Card
Note: Driver's Licenses and Passports will NOT be accepted