Early or late in the day and weekdays are less crowded than weekends (unless your trip is when local schools are out in which case the day of the week doesn't matter as much). Generally I have found that a day that is partly cloudy will have a lower attendance more than any other factor- a rainy day is even better for low crowds.
If you get there at opening you can generally ride most of the main slides in either of the water parks with little wait during your first 2 hours (I suggest starting with the bigger ones and working smaller), then spend the 4 middle hours of the day enjoying the lazy river, wave pool, and suh. The crowd will thin out again between 3-4 and then it will be easy to ride once again till close. Conversely if you want to take a break in the highest heat of the day and are going to Typhoon Lagoon you could bring a change of clothes, hop on a bus to DTD and eat lunch their and then return later in the day.