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A couple of things to keep in mind:
1) the depth guage meausres relative to where it is. If it is on the bottom of the boat, it is already about 6 inches down, so even a reading of "0" would really be about 6". Just something to keep in mind when comparing real depth to indicated depth.
2) Most depth guages can't measure anything under 3 feet. The depth gauge works by emitting a sound "beep" and then listening for the return. In order to work at greater depths the transmitted "beep" needs to be as loud as possible, and for better signal processing it wants to be of some reasonable length. While the "beep" is being transmitted, the receiver is getting blasted and can't possibly hear an echo return. If the water is so shallow that the echo comes back before the transmitter has finished and the receiver has had a chance to recover then it can't be detected. It seems that the "beep" lenght is about 3 feet.
I wish that there were depth sounders specifically for shallow water. For the ski boat, I really don't care about the depth once it is greater than 20 feet or so. I really care about the difference between 3.5 feet and 3 feet, however.
The answer to your question is: it depends.
If you are on plane, you can skim over a shallower spot than you could if you were at idle because the boat is riding higher in the water. The worst case is when you hit it, and the stern digs in a little before it starts to plane. You may have been fine at idle, but when you hit it the stern sinks in and your prop hits bottom.
As you start getting close to the bottom you will start to stir up an incredible amount of dirt and/or sand. Your prop may not be touching yet, but the wash of water coming off the prop is reaching down deeper and making it look like you are.
If the bottom is soft mud you can often survive a mild grounding. Any contact with rock can get expensive.
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