A couple of other things, if there is a thunderstorm all Dolphin swims are postponed until the weather eases, so your start time may be delayed if there is adverse weather (as we found out when we had a 2 hour delay).
Personally, I would be wary about visiting DC during either the early months of the year or later also, we found the water freezing cold last March and to be honest it did spoil our enjoyment.