If you visit one park, it is Waterbom — there is no real contest. Book online for the discount, arrive at opening (9 am) to ride the big slides before queues form, and rent a locker. Gazebo rentals are worth it for families staying the whole day.
Choose Splash only if you are staying in Canggu and want a park without crossing town — Kuta to Canggu traffic can eat an hour each way. The Finns day-pass combos make it decent value for a family.
Circus Waterpark is the budget pick for children under 7 who just want splash zones; adults will be bored.
Skip water parks entirely in your first two days if you are jet-lagged — sun plus slides is draining. And if you must choose between Waterbom and a Nusa Penida island tour, take the island: you can slide anywhere, but Kelingking exists only in Bali. More trade-offs in the Bali guide, and see water sports at Tanjung Benoa for the ocean version of a thrill day.