I went on June 28th and it wasn't bad at all. There were no waits at all until noon. After that we had maybe ten minute waits tops for Talon and Steel Force, but that was because we were being picky about our seats riding either front or back, otherwise it would have been less. If it's a warm day, the wet rides will probably have wait times around a 20 to 30 minutes at the most. The waterpark is another animal. Even though it was FREEZING that day, (Okay, it was around 73, which is freezing by Texas standards especially if you're going to be getting into water) there were lines for everything, even the lazy river. The water was so painfully cold that we didn't stay long anyway. To make a short story long, lol, you'll be fine on a Sunday.
...notice I didn't say "walk on"