I don’t know if they offer 6 day tickets? They used to have 7 day passes but thats been years ago (the last time we bought those was before 2010 and Harry Potter).
After 3-4 days the Annual Passes are usually around the same price. Depending on your dates, if you are only planning on one water park day, it may be cheaper to get the base level Seasonal pass for just the two parks and purchase a one day ticket to Volcano Bay.
Of course if you are entertaining a second trip and an additional visit to Volcano Bay price out the 3 park pass.