I'm pretty sure you can buy a one-day water park ticket and convert it (with credit for your single ticket) into an annual water park ticket. I believe it has to be done same day, at the water park.
You are allowed to leave the water park during the day and return to the same water park later in the day. However, you can't leave one water park and go to the other water park in the same day for a single admission. That's two admissions.
Edited to add: You could drive to the water park, park your car, then take a bus to a resort and transfer to a park bus. However, after 5pm (approx water park closing) no bus will stop at BB to drop you off to your car on return from a park visit.