Hi Amy,
I love cruising in September, the weather is nice and warm and the crowds tend to be a bit lighter because many of the kids have headed back to school.
Castaway Rays Stingray Adventure is an amazing opportunity to get up close and personal with these fascinating creatures! Most guests will opt to leave their personal belongings on the beach while they experience this adventure. The beach will be in full view while you are in the water, so you should be able to keep a distant eye on your things. If you choose to leave your things on the beach, you may want to consider putting anything that could be damaged by water in a ziploc bag for protection in the event of rain. Personally I wouldn't hesitate to leave my things on the beach, but keep in mind that Disney will not accept responsibility for lost or stolen items, and the safest place for valuables is in your stateroom safe.
If you prefer to keep your key to the world cards and identification with you, there are waterproof cases available for sale in the merchandise shops. Personally I would opt to bring a disposable underwater camera for use on this excursion. This way you won't need to worry about water damage, but can still catch all of the fun. Most that I have seen come with a band so you can secure it on your wrist while you play.
I hope this helps - have a wonderful time!