It's simple, you don't need a cop at every flooded road.
But you should have a cop attend every vehicle that is stuck in flood waters and book them for negligent driving.
It should be a strict liability offence.
The danger is obvious so thus they are negligent.
If someone is injured the offence should be higher.
Just because someone decides I am now 'offroading' doesn't mean your actions are safe nor legal.
Fining people for a clearly high risk act is entirely reasonably.
We have laws fining people for simply not locking their car, which is purely a cop out for insurance companies. This is about safety, so I don't think a water crossing fine is extreme.