Hi John,
You won't
need the Park Hopper Option on your tickets to visit the Magic Kingdom and Typhoon Lagoon on the same day. The tickets that you purchase will be for a number of days to visit the parks.
To visit the water parks, you will need to have purchased the
Water Park Fun & More Option add-on to your tickets. The add-ons are for a number of admissions (based on the number of days on your tickets) for the water parks, DisneyQuest, ESPN Wide World of Sports Complex, and/or a round of golf at Disney's Oak Trail Golf Course. These add-ons will be in addition to your park tickets.
However, if you want to visit only Typhoon Lagoon, it may be less expensive to purchase admission separately to the water park.
Thank you for the question; please let us know if you have any more questions.