Order a PRC7925.
This is for the Range Rover but fits the 300TDi.
Th stalk has a reed switch that is triggered by a magnet. When it arrives, the reed works the wrong way round - that is a high water condition shorts the contacts.
What you want is the other way round - high water keeps contacts open. So you pull the float off the end, and then put it on the wrong way up. The keeper plastic disc on the bottom can then be melted back on with a dab from a soldering iron.
Now you take the level sensor for the braking system and you tap each connector wire so it also connects across the water sensor. Then - if the water or the brake fluid gets too low it will light the dash lamp and you then get an immediate stop warning.
Because I'm a little nervous of missing a light I also have a 12V buzzer wired across my oil pressure and brake lights - so any problems and the buzzer also triggers.