Check with your hotel, many have shuttle service to the Theme Parks.
Some are free and some have a reasonable cost depending on your hotel, but all run on a limited schedule that may (or may not) be convenient to your needs.
Remember, these are shared services , usually with several other hotels in the area so you are required to book in advance and expect there to be several (to many) stops on the trip to and from the parks.
As someone mentioned above, if you are staying off site then a rental car really is an asset.