Buy yourself a chart of the waters you plan to boat on. You'll find nautical charts at any marine supply store. Purchase one that is waterproof, easily read and current.
Ask the people at the store or at the marina how accurate the local nautical maps are. There are places in the water with sand bars that rise and fall according to the seasons. Get as much information as you can before heading out.
Insure that your depth finder is working. Remember that that the device may not be accurate if you're breaking plane and speeding down a channel. You must be going fairly slow for a depthfinder to work correctly.
Watch the buoys when you navigate a channel. The old standby is "red on return". That means stay to the left of the red buoys when coming into port, and make sure the green buoys are on your left going out. Stay between the green and red buoys. That's where the channel is in shallow water.
Cut the engine or put it in neutral if you feel or hear scraping so you don't ruin your props. If you're in shallow water and your boat is too big to pole out, raise your props up and try to back out. If you can't raise your props--for instance, if you have an inboard--anchor and call over your radio for a tow if you think you'll run aground. If you know the area and you feel you can back out to deeper water, try it. You might want to jump over the side and dive to see how deep the water is, weather permitting. Don't go over the side unless the boat is anchored and you have someone else onboard.
Watch for lighter water. Lighter water usually means that the water is more shallow. Also watch for "deadheads," which are partially submerged trees or logs. Hitting one of these at high speed could destroy both you and your boat.
