The Southern California area doesn't get floods often (not talking about the desert area, you guys know).
OP is completely correct. Things that regularly show up in flood water:
Raw sewage from pipes or septic tanks that got damaged
Staphylococcus, which when it takes root in any hangnail or scrape, or heaven forbid inside your favorite orifice, is a potentially dangerous infection
Ants. They don't drown easily and when they find higher ground, for example up the leg of someone wading, they take advantage. Wolf spiders and fleas do this too. And trust, if it's a city area that flooded, there ARE fleas.
Water that is deeper than you think, which can absolutely swallow your truck. You might know what the ground level looks like when it's not flooded, but you don't know what got washed away, or softened.
Glass, metal, branches, wire, really the whole gambit of things you don't want to be inside your skin. Can you see clear to the bottom in that water? No you cannot.
Dead things. Rats, stray animals that didn't make it to safety, fish who choked on the muddy water (if we're talking about a river or creek that burst its banks). Would you get into a swimming pool if you saw even one rat floating around in there? How about fifty?
Seriously friends, be safe. Don't go canoeing in the street no matter how funny it is. Don't drive your truck through the big puddles to make it splash. Don't go swimming. If you really have no choice and MUST enter the water for whatever reason, get out quickly and wash with clean water immediately, especially your eyes and any abrasions. Hospitals and the rescue aids are already going full throttle, don't add to their workload by being dumb.