Okay, we don't mean that literally, but if you see water flowing across the road from one side to the other and don't know how deep it is, don't try to drive across it! Let's say that again in case the rain was too loud for you to hear it the first time: Don't try to drive across it! Every year hapless drivers figure their cars won't be harmed by fording a tiny little stream of rainwater that couldn't be deeper than, oh, an inch or so. You'd be surprised how many of these people then find themselves swept away, off the road and into a vicious current of rainwater that could carry them for hundreds of feet as they desperately try to get out of their car and grab a tree limb to keep from disappearing below the waterline. Better to wait out the storm and the rainwater than to lose your life -- and quite possibility the lives of those near and dear to you -- because you thought a little water couldn't hurt you.
And even if the water isn't moving, if the bottom isn't visible you don't know what's underneath it. There could be a pothole the size of a swimming pool. There could be broken glass or nails that fell off a truck. If you can't see the bottom, don't risk putting your tires on the submerged pavement -- or your life on the line. Either stop a safe distance from the water's edge or, if possible, find a way to drive around it.