Hi Jaylie,
Another option is to buy just the water park AP. That's what we did last year. We took our first trip in May, and didn't have time to go to either WP, went back in August, bought the water park passes, and used them every day.
To upgrade our AP's to PAP's it would have been the full amount. To just purchase a water park AP it was around $100 per adult, and $80 for our DD. By doing this though, you don't get the Disneys Wide world of Sports, Disney Quest, Pleasure Island admission though. That was not a problem for us.
Good luck!