Hello! At every resort, you can simply go to the bell services stand right outside the front doors, and the staff there will get a cab for you. There are usually cabs waiting, but even when there aren't there are SO many cabs around on property that you won't need to wait more than a few minutes.
Never been to Universal, but it's usually around $30 for the cab to SeaWorld. Once you leave SeaWorld, there is a cab stand just past the entrance -- and again in our experience there have ALWAYS been cabs sitting there. Also, depending on the time of year, if it's a slow time (and if you like the driver you get for your first cab), drivers almost always offer you their card and direct phone # and are more than happy to come back to get you at the end of the day.
Have a great trip.