In this case I would do a new merged layer above all your other layers (Cmd Opt Shift E). Then do the Liquifying on that layer.
BUT, here's the important trick. After you've done the liquifying, but BEFORE pressing OK to exit the Liquify window, hit "Save Mesh":
Save the mesh file with the same filename as your photo, so you don't lose it.
After saving the mesh, press OK to commit the Liquifying.
The reason we do this is in case you find you need to do other editing with your Levels layers or whatever. In that case, you'd need to delete your Liquify layer, you see? But that's ok, because you can re-do whatever Levels work needs doing, then make a new Liquify layer on top (Cmd Opt Shift E again) then go into Liquify filter, hit "Load Mesh" and restore the liquifying you did before. No need to do it over again.
Is this making sense?