i'm quite sure water does not "moisturize" or help the rubber seals in any way. if anything, silicone grease would help. to check your BCD, just orally inflate it and leave it overnight or a few days to check for leaks. Tug firmly on dump valves and elbow joints to check for possible failures. the part that might need servicing would be the inflator. you can take it apart and clean it if you know how. otherwise you'll find out when you connect it to a tank.
to make my equipment last longer, i try not to leave them out in the sun to dry. the rubber and plastic parts deteriorate faster due to UV exposure.