For a forceful, powerful splash:
* Like a bomb exploding in the water.
* Like a giant hand slapping the surface.
* Like a thousand tiny hammers hitting the water at once.
For a light, gentle splash:
* Like a feather landing on a still pond.
* Like a soft rain falling on a windowpane.
* Like a whisper in the air.
For a playful, joyous splash:
* Like a child laughing and jumping into a pool.
* Like a dolphin leaping out of the ocean.
* Like a burst of sunshine on a cloudy day.
For a disruptive, messy splash:
* Like a paint bucket being overturned.
* Like a wave crashing on the shore.
* Like a mudslide running down a hill.
The best simile will depend on the specific context of the splash and the emotion you want to evoke.