As with most water parks. I imagine lockers will be provided near the front of the park for a daily fee (and maybe a deposit fee you will get back at end of your rental.)
At most water parks many slides allow (waterproof) cameras, as long as they're small, and preferably strapped to your wrist (GoPro style.) This may vary between rides.
Bags or backpacks are never allowed on slides.
Sunglasses/Glasses are usually allowed on slides.
Lockers should be rented, or cabanas, so that you have a safe place to put your phone, wallet, towels, bags, sunscreen, and any other items you will not be taking with you on rides.
It is most likely that selfie sticks
will not be allowed into the park at all.
To carry money or cards with you, even on slides, I'm sure waterproof plastic containers may be allowed on most, if not all, attractions. They may even sell their own in the park:
If you pack light, you may not even need to rent a locker for the day. Some water park visitors are brave enough to leave a bag unattended on a chair near the wave pool while they ride slides, but I would advise against that.
I do not believe there will be lockers near rides, but there may be shoe caddys to hold your swim shoes and flip flops while you ride. Universal will not be responsible for shoes/items stolen from the caddy.
This will not be like the two theme parks. They will not have free lockers at every ride. This is a water park, and it'll be up to you to keep track of your stuff. A locker usually cost like 10 bucks for the day, and that's a good deal for piece of mind.
(I may be spending too much time answering questions today.)