Sorry but it’s really not much extra.
The cost between a multi-day park ticket with park hopper in comparison to the same ticket with water parks added as well is only $20 US.
This is part of the reason we’ve added WP tickets the past few trips. We normally traced during free dining so are required to purchase tickets with the park hopper option. Adding the WP option isn’t too much extra for all the fun it adds. It also adds a round of mini-golf toi. A great deal fir $20!