Ping-pong render targets, rgba16float, wave equation
Two floating-point textures swap roles every frame. The red channel holds the surface now, green holds where it was last frame, and that second channel is the entire reason a wave keeps travelling rather than snapping flat. Each step is the discrete wave equation over the four neighbours; the shading pass turns the height slope into a normal and lights it. Drag across it, or leave it alone and it rains on itself.