A couple days ago I went with a few friends to the Salt River to go tubing. It was my first time and I learned several tips on how to be safe:
1) Go with a couple people. They will help remind you to do things, like put on sunscreen. Also, they can help you paddle in the right direction when there is a strong current.
2) Use spray-on sunscreen (85 SPF) and a sunscreen stick for your face. I used both and I barely got burned.
3) Remember to apply before going in the water, 30 minutes after getting in and every hour after that. I followed this mostly religiously and didn't get burned, except for a couple small places.
4) Don't go in the water too often or apply sunscreen again right after you get back in your tube. My friend went in the water a lot and ended up getting badly burned.
5) Bring a hat. I couldn't find my hat so I didn't wear it and ended up getting burned on my scalp. It is really itchy and irritating at the moment...
6) Bring water shoes. I only brought flip flops and it was really painful to walk on the rocks until it was shallow enough to put on my flip flops. Also, in case there is something sharp in the water, water shoes are safer to wear than going barefoot.
7) Bring lots of water and snacks. There is no shade, except under the trees at the side of the river (where snakes supposedly linger). Therefore, you need to remember to bring tons of cold water to stay hydrated and prevent heat stroke. Also, a few snacks and lunch will sound good, especially if you take the longer routes.
8) Bring an extra shirt. If you start to feel really burnt, a light t-shirt will save you from the sun a little.
9) Wear a one-piece bathing suit for more protection. I wore a one-piece bathing suit and shorts to save myself from any unwanted sun.
10) Bring an umbrella if you can. I didn't bring one, but that would definitely improve the not-getting-burned situation.
11) Remember to bring a cooler and an extra tube hold the cooler and other belongings. When I went, my friends brought a cooler and three extra tubes (two small ones and one big one for the cooler), to carry food, drinks and personal belongings.