Think of it in terms of DAYS and PLUSSES.
A DAY gets you into the 4 theme parks. You cannot use more than one DAY on a pass on the same day, so if you want to get into more than one park on the same day you must have the Park Hopper option.
A PLUS gets you into one of the water parks, DisneyQuest, or Pleasure Island. You can use more than one PLUS per day, and you can use PLUSSES on the same day that you use a DAY.
If you plan to have 7 days in the theme parks, and 3 days in the water parks without going to the theme parks, then a 7-day pass will be more than enough (the 7-day pass comes with 6 PLUSSES).
Think about this, however: the price difference between a 7-day pass and a 10-day pass (if you are not adding the no-expiry option) is less than $10. That $10 would give you extra DAYS that you could use to enter the theme parks on all 10 days of your trip, even the 3 days that you plan on going to the water parks - thus allowing you to spend a few hours in the parks, have dinner in an in-park restaurant, watch the evening parades or fireworks, or just ride a few rides, after your day at Blizzard Beach or Typhoon Lagoon.