When we go to the water parks, we leave our valuables/wallet etc behind in the room safe (or the safe at hotel front desk). We put our tickets, room key, and a small amount of cash for food etc into a small ziplock bag.
When we get to the park we rent a locker to store the ziplock bag, towels, clothes, shoes, sunscreen etc. I think the Disney ones were maybe about $7 with a refund of $2 when you bring back your key. When we went in 2004 they gave you the choice of a free plastic mug or the $2 refund.
You just put the locker key around your ankle or arm, the same as other waterparks.
You can rent lifejackets for kids, but you don't need to rent innertubes at Typhoon Lagoon or Blizzard Beach. They are supplied at all the rides, wave pool, and the lazy river.
I think I remember at Wet n Wild on International Drive that the only thing you needed to rent innertubes for was the wave pool, although you could just enjoy the pool with no tube if you want. All their other rides had tubes supplied.
At Typhoon Lagoon, if you want to snorkel in the saltwater pool with the sharks and fish, do this first thing when you arrive at opening time because the line can get long. This is totally free. You can also walk down into a separate viewing area where you can watch them under the water through windows.
Do try to get to the waterparks by the opening time, as you can get a lot of rides done before the parks get too crowded. Remember that weekends are busier than weekdays because you have more local people.
They have park maps at the entrance, which you can store in your locker so they don't turn to paper mache in your pocket.

There are signs around the parks with big maps, so you can always check on which rides you still need to do and where they are.
Don't forget to take hats, sunglasses and sunscreen. Put the sunscreen on several times during the day. The first time I went to Typhoon Lagoon I got sunburned so badly on my legs that I could not even walk properly the next day.

Even on overcast days you can still get burnt.
Lastly, if there is a thunderstorm in the afternoon, they will clear everybody out of the water and you have to wait around until they say it's safe to go back in the water. Don't leave the park. Just wait it out - sometimes it's only for about one hour or less. Some people will leave the park when it starts raining, so if you stay you will hopefully have less crowds to deal with.
