Your best plan is to purchase a single water park entrance at the entrance to the Waterpark ($38 for an adult vs. $50 for the WP Fun & More Option on your Magic Your Way ticket) .. and perhaps a two day Magic Your Way ticket to the major parks.
You will pay near top dollar for a two or three day ticket because the tickets are priced such that additional days beyond 3 are quite cheap compared to the first three days.
That's just how Disney does their ticketing.