Presumably the sign refer to the depth of the creek (river?) over which the bride passes; thus not really a good indicator of depth of water on sed bridge. Which having said.... the way some of the drivers approached the flooded section leaves one wondering if they have any idea how to negotiate "puddle"" and the like; especially in a typical family/smarller car. Trucks etc usually have a higher cleraance too and the engine less vulnerable to being drenched; not that this didn't stop some heavy trucks etc. stalling...And then one has to ensure the exhaust tail pipe doesn't get flooded which can cause an engine stall ... if driving into a flooded area at too slow or too a high speed; orif it doesn't happen due to water inrush at the front onto the engine causing it to stall and thus exhaust pressure drops... and then the engine simple chokes to stop. My own experience has always been to get out of vehicle and check, before driving through something that looks a little dubious in terms of depth, and poke around to see how deep it may/may not be... But then that's usually been in in another country and out in the wilds... Or else... turn back and find another route clear of the problem - if possible.
There is ford (I think it's in Suffolk) where the locals regularly see drivers ignore signs about the ford being " a little deep" and not really wise to chance much of the time; yet they persist and the local tractor owners do very nicely as a result... Doesn't help that satnavs seem to give it as part of a route good to use....