Yes, the plus is an extra ticket. Instead of thinking of it as a single ticket, think of it as three tickets, each one worth a day at any of the four parks, PLUS a day at the water parks or Pleasure Island. If you plan on going to PI and a water park, use the plus at the water park. The PI tickets are out there for about twenty bucks, the water park thirty. Note: each option is good for only the day you use it. If you spend half a day at the parks, then go pop in a waterpark for a minute, you've activated your Plus and it's only good that day. But the 3 day plus should give you four days of entertainment; three at the parks, one at a waterpark.
Pat