Short term metered water vapor injection can be used for a quick power boost but the engine needs to be properly configured. A modern computer controlled engine the gain might marginal or even result in a loss of engine power without reprogramming. Many fighters and heavy bombers during WW2 used water injection for short term boosts during take off with a heavy load or during dog fights.
Un-meter rain water ingestion into a modern day diesel engine? I suspect it is something best avoided. I'm a bit surprised that who ever rebuilt the engine didn't go thru the intake and such searching for a cause of the water such as a clogged drain.
