Forgers use digital thermometers, aquarium thermometers, and infrared thermometers. There's usually some thermal stratification, so the water at the top will be warmer than the water on the bottom, so you'll get higher readings from floating thermometers. Another way to think about water temperature is this:
Fresh water freezes at 32F, so if you've got ice in your Forge, your water temp is close to freezing! I often wonder if some ingenious Forger is working on an underwater, WiFi or bluetooth enabled wireless system for continuous monitoring of temperature. I'd be interested in that!