Of all the tutorial videos on water simulation, they're all like water flowing from a point and then settling. How would I do it if I want the start of the animation to be an existing body of water. Would I just do a flowing animation, wait until it is still, then just cut the flowing part of the video off?
You can use an Object set to a Flow Behavior of Geometry to convert any part of the mesh that intersects with your domain to liquid particles. You'd just have to allow a limited amount of time to settle and keyframe the visibility of the object.
Particle sims, and particularly this process, seem pretty finicky right now. I had to go back and change the number of divisions on my domain to sort of "refresh" the sim and update the size and positions of my emitters. Just a heads up.