Easiest way of thinking about the MYW tickets:
For the four theme parks, think of a 7-day MYW ticket as being for 7 *calendar* days. A base ticket will get you into one park per calendar day. In order to go into more than one park on any calendar day, you must purchase the Park Hopper option. No other way around it, unless you purchase an entire second ticket (which isn't cost-effective)
Now, the Water Parks & More option works in the exact opposite way. The WP&M can be thought of as *admissions* and not days. You could theoretically go to Blizzard Beach in the morning, Typhoon Lagoon in the afternoon, and then an evening at Disney Quest. Doing all this would require you to use *three* WP&M option admissions.
-Rob