I would imagine that at the end of August in Florida on a Saturday, you may find the waterparks to be more crowded than is strictly comfortable or relaxing. It's a personal thing maybe, but I have a pretty strong threshold for the amount of bathing suit-clad people I can take in a confined area. Weekends are more popular for locals to attend the parks than weekdays.
AK is your best bet for a "slow" park, IMO. Plus, depending on the date in question, HS may be mobbed as Galaxy's Edge opens on August 29th.