If you want to visit both the themeparks and the waterparks during your 8 days, why not opt for a 5 or 6 day MYW pass with the water park option instead of an 8 day base or 8 day w/waterpark option? This is what's so great about MYW tickets, you don't have to buy tickets for the number of days of your visit.
The 5 day comes with 3 plusses, 6 days offers 4 plusses (waterparks, Disney Quest, Wide World of Sports, Pleasure Island), so if you're planning 8 days without hopping you could do one activity each day with a 5 day premium pass.
Can you modify your itinerary and buy fewer days? Compare the prices with the options to your schedule, see what works.