Fluidity HD is a wonderful free sensory app. When you swipe your fingers across the screen you move the fluid into an array of colours. This app is a bit like a lava lamp at your fingertips. Children love it. It works on iOS.
Fluid Simulation is a similar app available for Android devices.
Pocket Pond 2 is a really popular free app. It is basically an interactive fish pond. When you touch the screen you can hear the water splashing and fish swim off. You can add lilypads and noisy dragonflies. It is available on ios and Android.