Water

Tyres have grooves to suck rain off the road.  This "tread" helps them grip, like the sole of a shoe.  But standing (or running) water, or a flood, can cause problems for all road users.

In short: never drive into water you couldn't walk through.  You might get stuck.  Water can hide dips in the road, or other débris which could damage your tyres or steering.  The road itself may be washed away.
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150 mm of water can cause a car to float, or to stall as cold liquid touches hot parts underneath.  You may lose grip, or be swept along (or under) by even slow-moving water only 300-600 mm deep.

If you must go through, take the middle of the road.  Water is shallowest here, and débris less likely.  Watch other drivers first, to see how they get on.  Pass through one at a time.  Don't drive through water while another vehicle's coming.