Usually, the weather is the best (sun is bearable in the morning vs noon time), you can find nice seating to put your stuff down, there are shorter lines, and knowing Flordia, it usually rains around 1pm to 3pm almost like clockwork. Going as the park opens allows you to catch everything before 3pm, have a nice lunch, and allow you to have a relaxing early evening.
As for the winter months, its hit or miss because usually Disney has one water park down, but going early has always served me well!
Walt Disney World
Tokyo Disney Resort
Theme Park Insider Books