We are in GA also and have the first week of April off for Spring Break. I have been during this week before and found that while the parks are not mob scenes, they will still be very busy and you should have a good plan before you get there and book any dining ahead of time for restuarants you do not want to miss or character meals. The last time, I felt the weather was alittle cool for swimming, not cold, but too cool for my southern blood! My son however did not seem to mind and swam at the hotel. You can never tell though. We were just in Orlando for Presidents' Week and the weather was in the 80's and you could definitely swim or go to a water park and be comfortable until very late in the day. Not sure if we will be down in April, but will definitely be down for Memorial Day week.
As far as hopping, I would definitely upgrade if you can. It is so worth it to be able to hop to another park if you want. We almost always start off at one park and finish the day at another.