One option might be to rent at the Disney Care Care Center. They will pick you up, take you to the center, then you get your car, go back to the hotel and off to Sea World. You can also rent at the Dolphin and I think at one of the Downtown Disney hotels.
Taxi is also an option, tho I expect it could cost more (perhaps someone can price this out for you). The Town Car services will do this as well, tho they might be pricier than a cab (tho plenty more comfortable).
Good luck!