When we were there in May, we were only give a few times to choose from and they were in the morning. I think the earlier you arrive for check-in, the earlier the dolphin swim times you can choose from. You may be able to request a later time, but we perfered the morning. If you do it in the morning, you don't have to worry about checking the clock all day and worry about missing your reservation, and you don't have to worry about an afternoon thunderstorm interfering with your swim.