How to make a vertex water displacement on a full cubic ocean?

Hey! I'm making a puzzle with tiny cubic worlds, like that :

As you see on this screenshot the "ocean" is cubic! i would like to apply a vertex displacement simulating little waves.

What is the best solution for that? is there already something in Unity or from the asset store i can use? or should i make or find an original script for that?

thanks for your help!! i appreciate it!! ^_^