As far as my experience is concerned - I made a critical total assessment of an existing, large firewater pump installation at a major LNG plant last year - you may not be legally allowed to use electric motor drives on your firewater pumps. You should double-check with your local Fire Marshall and your Insurance carrier.
The reason for employing diesel engines and not electric motors is quite simple: in a fire case, the electric substation or the power lines could be affected either before or during a major fire within your facility and the consequences would be not only embarrassing, but also fatal. Unless you can prove that there is no way that the electric power can be affected by a local fire, I would never rely on electric motors to deliver all the required fire water during a fire case.