I'll be finally hitting this in September. Any recommendations?
Try to arrive early as Shiekra mentioned.
Always be in queue for something. Don't worry if it says to return, it will not remove you from the queue. Also, they will let you ride at "5 minutes", you don't need to wait for it to say return.
Grab chairs right away. There's free and paid seating all throughout.. I personally prefer the free seating near the back of the pack, just past Ohno and Ohyeah (when you head left after arrival). There you'll find a shaded spot which is generally pretty secluded. However, if you're planning to suntan with a good view, you could always complete for a seat in front of the volcano, but expect a lot of sun and people.
Unless you plan on spending lots of money on theme park food, I highly recommend packing your food. You can bring cans and plastic bottles, no glass and no alcohol. (They might inspect your drinks to ensure they're not glass or alcohol.) You can bring meals and snacks, I usually bring sandwiches +plenty of chips/fruit/etc.
You can bring in soft coolers, no hard coolers. I bought a soft cooler backpack (about $20 on Amazon) which I pack full of cooler packs, drinks, and food, which I also bring to other theme parks.
Waterparks can be exhausting. I bring coffee/energy drinks to keep me going.
Drink lots of water. There's drinking fountains around the park which are above par. Or bring your own.
Bring and use lots of sunscreen. Even if it's not too hot or sunny, still use it. I've seen plenty of people burnt and it's just so unnecessary. I recommend buying a cheap sunscreen (like Walmart brand) for your body and a mineral sunscreen (zinc/"no tears") for your face... Otherwise your eyes may burn. (Do your face first, then body... Otherwise the body sunscreen will still be on your hands.) Apply multiple times daily, especially to spots like your shoulders/neck/arms. Don't forget your ears.
I like to bring a change of clothes to wear after using their (free) showers at the end of the day.
If you bring your phone, I recommend buying a cheap waterproof case (I got one on Amazon, 2 for $12 I think). You can't take it on a couple of slides (the drop slides/body slides), so either get a small locker or leave it with a friend/family members who isn't going on that slide. (Even if your phone is technically waterproof, beware of sand.)
Larger lockers are generally not necessary. Small locker for possible phone or money, but I don't see a need for anything else. People generally leave their stuff at their chairs, covered by shirts/towels. (Reconsider if you have luxury items I guess.) Lockers are reusable, you can open as many times as you want throughout the day.
You can leave small items on the net-shelves outside of the rides, items such as sandals, sunglasses, shirt, etc. Most slides allow sunglasses. The body slides/drop slides don't allow any items besides prescription glasses (which you'll need to hold in your hand). You can check with the Team Member prior to entering the ride on what's allowed.
The walkways can get hot on sunny days, but they spray the floor with water in most areas... Walk where it's wet if you're barefoot.
If you're going with a family, plan on seeing a lot of skin. They don't seem to have strict rules on this. Go to Disney if you have a problem with that. (Assuming Disney has fewer thongs, though I haven't been there in a while.)
Unless you don't want spoilers, watch some walkthrough videos on YouTube (such as from Tim Tracker). They'll give you some other tips and you'll get a better feel for the park.