Unfortunately a depth gauge is of limited help because the depth in the flats can change quickly. Study your chart, get to know the low areas. Ask at the local bait shop if there are specific areas where you should be extra careful. Understand the tide charts; not just high tide and low tide, but how high and how low when you will be on the water.
A couple other safety suggestions: Don’t drive 80 mph over water you’re not sure of. Wear a lifejacket. Attach the engine kill switch to your body.