If you habitually leave your retractable keel down (which is recommended by most boat manufacturers), then it will warn you when you are in shallow water. You'll feel it dragging when you are underway, and you'll feel a change in the boat's motion when you are at anchor. If it is down when it touches, then you can crank it up and get free. If it is fully retracted when it touches, then you won't get any warnings until the boat is hard aground, and, since the keel is already up all the way, you won't be able to crank it up any further to get free. IMO, it's a good practice to leave it down, and only raise it if you have grounded. If the keel is down, and the boat grounds on a falling tide, then by cranking it up you will gain some time to move the boat to deeper water.