Yes you can install near the water heater switch outside the bathroom. There are small casings that are meant to fit a single RCCB (2u size)
The other choice is to get a RCBO, and replace the MCB for the water heater that is in the DB. RCBOs are the same size as MCBs, and are meant to swap 1-to-1. RCBO are dual function devices, an MCB and an RCCB in one unit