We have rented from the airport before and then dropped off at the Dolphin. We use National.
Last time we went, we had a free one day rental and ended up renting from the Dolphin for one day to get back to the airport instead of using ME. The problem with getting a car there is that the selection is very limited. We got a pretty crappy car that didn't even have power windows or locks. It is better to get the car at the airport because the selection is much better and then use the Dolphin as a drop off. The counter is located just inside the hotel and the valet usually takes it and parks it for you.