well, I think you want to mirror the top half onto the lower half if you want the sky and the moutain tops to look like they are mirrored on the surface of water.

Either way, depending on what fps you need, the this method might work:

create a PG graphic image of whatever image is displayed in your draw window, eg. PGimage.beginDraw() & PGimage.endDraw() for everything you want reflected in the top half of the image,

then display after  in the same draw() cycle, or the next, the pg graphic image you've created with the begin and end draw statements, which is the same as the top half of the sketch window, plus it's rotatations and whatever other effects you might want to apply, like a little blur and probably some sort of perspective distortion ..well apply rotations and viewing angles of the PGimage to suit whatever reflection angle you desire.

In OpenGL you can use viewports to identify the which part of the sketch window to render to.

This link has some code which splits the display horizontally into two and renders the image twice using different camera settings. To mirror the image you might try change the 'up' vector on the camera from 0,1,0 to 0,-1,0

doing this was a total bitch, however it works: