The primary problem with heat pumps is the steeper the gradient is, the less effective the thermodynamic cycle is at pumping the heat. What you're envisioning is locking both sides in a very steep gradient and throwing any semblance of thermodynamic efficiency out the window. The more heat you pump out of the refrigerator and into the water heater, the less there is to pump. Similarly the more you pump into to heater, the less you transfer. You could do a multi stage thing that switches to a third environment to make up for this, but since you're not taking a shower at the same time you're opening your fridge you're unlikely to see much gains by linking them. And if they had no third environment at all, well you'd be having that compressor trying for impressively low temps in your freezer after a shower. And avoid going full hot water after a grocery trip if you don't want to get burned.

This stuff can be done on a commercial scale, but it's more nuanced than just linking the condenser and evaporator coils.