I wouldn't leave a floating thermometer in a brew pot in case it breaks. However, I have one that clips to the side of the brew kettle (http://www.midwestsupplies.com/dial-thermometer.html
). I leave this in from when I start heating up my water until I dump the wort into the primary.
On another note... water boils at different temperatures based on elevation above sea level. 212 degrees F is for sea level. For San Antonio, elevation of 772 feet above sea level, water boils at 210.5.