Hi Dana. I have been trying to find a link for a similar question. WDW Unofficial Guide has a great little chart rating "busy," "moderate" and "light crowds" for days at all the parks. I can't find the link. Anyone?
I don't think Sunday was one of the "busiest" days but in the summer the water parks are pretty much ALWAYS busy and you have to get there fairly early as they often close due to reaching capacity. You can also go later in the afternoon but unfortunately the current summer schedule has the water parks closing at 5.