First thing in the morning is best, or after a thunderstorm.
They have to shut down all the water attractions and pools during a thunderstorm. During summer here in Florida, there is a thunderstorm almost every afternoon. At the first sign of thunder, the water parks clear out....FAST! If you go after the thunderstorm, you can walk right onto everything. We arrived at TL during a thunderstorm a couple weeks ago and everyone was pouring out of the park. When the park reopened 20 minutes, later, there were probably no more than 100 guests in the entire park. It was walk right onto everything, the lazy river had nobody in it, and we could snorkel to our hearts' content at the shark pool.....we were the only ones there for at least 15-20 minutes.
Going for a couple hours in the afternoon works for us because we have AP's and live 10 minutes away. If you are doing it as part of a vacation and are paying full price to do the park, get there no later than 8:30 for the 9am opening.....stake out a good spot in the shade, and hit the slides right away before the lines get too long. You can do the wave pool and the lazy river later in the day.... they don't have lines.
The park usually fills to capacity by 10-11am, so get there super early. You have to walk from the parking lot, and the earlier you get there, the shorter the walk.