Just thinking about how that would have to be coded and then how it would tax the machines made me cringe a bit. It is a nice concept, but would be awful to implement, so I completely understand.
Thank you for even considering it momentarily, though!
Why would it be expensive? Water has constant changing/morphing reflection and this could be ‘static’? That shouldn’t be too expensive in terms rendering power.
The portals aren’t ‘too’ expensive either since you either see a very narrow part of the other world or a very little part of your current world.