We just stayed at Pop at the end of August (my 12ydd and I) and did one day at DC and one day at Sea World.
For DC we booked the Mears Shuttle the day before. They had set pick up times, 6:25, 7:25, 8:25, all am. We got picked up at 7:25 in a van with another group. They said depending on the days reservations it might be a van or a bus. They said it could take up to 45 minutes to get there, again depending on how many pick ups. We lucked out and went right from Pop to DC. It was $18 pp round trip and in the afternoon we had made arrangements for the 3:45pm. I *think* the returns were like 1:45, 3:45, 5:45. You could call them 2 hours before you wanted to go back day of or we booked it when we made the initial plans.
We LOVED DC but by 2:15ish we were ready to go home (we got there around 7:45 or so, had our dolphin experience at 8:40 and had done all we wanted and then some!). So we opted to take a taxi back. There were some waiting in the driveway. It was right around $24 I think and then I added tip.
So I lost money (since I had paid for the shuttle round trip) but I did like the convenience of the taxi coming when we wanted to go, not having to wait for a certain time.
For Sea World we decided to just use a taxi. It was again right around $24 for the ride plus tip. Very easy, there was one waiting at Pop and for the return there was a string of them waiting outside Sea World.
Hope that helps!
Allyson