There are places where you can get the historical park hours (prob. here at DIS somewhere!). I was there last year the 3rd week of August (like Aug 16-23rd) and it seems like MGM closed around 8:30 and the MK's hours were varied. We had 2 eride nights - both early - one starting at 7pm and one at 8pm and those were awesome! Very much worth it. I know we were at the MK late one night for fireworks (maybe at 9?) and there was a 2nd spectro added for that night so MK must have been open until 10 or so. That got added after we were there I think so it may not show up on the official hours list you find on some websites. (This is my recollection - I could be wrong! But I remember thinking the 2nd spectro would have been much less crowded but we could not stay of course with 2 little kids!) The parks are not open as long that 3rd week and after but having those 2 eride nights more than made up for it! Epcot always seems to close at 9 after the fireworks and AK closes early - like 6 pm...but that is typical I guess.
As for swimming, it was VERY rainy in the afternoons when we were there. Not just rain but storms so the pools were closed most days in the afternoons for several hours. We ended up rearranging our schedule a bit to go to the pool in the morning one day just to get some pool time in for our son who LOVED the AKL pool. We did get in pool time every day somehow it seems. We did go back to the parks in the evening and have dinner and do something (see Fantasmic, Spectro, Illuminations). The problem in staying all day at the park is how hot it is that time of year. It depends on the ages of your kids perhaps (my 2 were 5 and 20 months old at the time). We did a long break every day at one end of the day or the other or in the middle of the day.
If you get eride nights take advantage! Those were the last eride nights of the summer (of the year too?) and it was well worth it.