Each plus is like a separate admission ticket to the minor parks. They are good forever until you use them. They have no relation to the park hopper portion of your ticket.
So, if you use a day's admission of your parkhopper, then decide to go to a water park, you are using one of your pluses in addition.
If you go to just a water park on any day, like you are planning for your seventh day, you will only use up one plus admission - no effect on your park hopper days!
The only head's up you may need is that the plus are NOT hoppers. If you go to one water park in the morning - then leave to go to the other water park in the afternoon - you will need to use two separate plusses.