We have been in both July and April and now choose to go in July.
Our general rule is that if the Q time is >30mins we dont wait.
We also go for 3 weeks which we couldn't do at easter. If you can get to the park at park opening then the crowds are not too bad for the first hour and then you probably get another hour when you wouldn't want to wait in the queues for the major attractions (>30 ins) - but with careful planning you will either have ridden them or have fastpasses for them.
By 11am the queues for all the major rides (perhaps with the exception of Duelling Dragons at IOA) will probably be over 1hr and may not go back down for the rest of the day.
I think the parks are busier at Easter so the queues build quicker and the times being given out from the fastpass machines are later. Also at easter we found even the small rides got busy while in summer it was mainly the major rides.
We aim to be leaving the parks at 12ish as we find that the children start finding the heat uncomfortable (but not as uncomfortable as when we get back in a hot car

). It then gets really hot and potentially very wet by mid-late afternoon - we are never in the parks at this time and the rain/heat rarely disturbs my shopping
The parks vary in when they are open until - you'll get an idea from the Disney website. We go back to the parks in the evening but dont tend to go near the major rides but rather do the quieter areas of the parks and take in the parades/fireworks. I personally enjoy the atmosphere of the evenings. Seaworld is a great park in the evening.
I dont know if we are unusual but no-one in our family finds the humidity a problem - I cant say I even notice it. I just love the warmth in the air
We used to do the water parks but none of us enjoy sunbathing and with little cover in the queueing areas we found it too hot from mid-morning. We decided that we would rather spend time around our pool at the villa. It seemed to me the queues at the water parks got longer earlier than at the theme parks.