I would use a k-type thermocouple and my Fluke DMM with thermocouple adapter. The FLuke is way more accurate than an inexpensive cooking thermometer, and traceable to the NBS if properly calibrated, but only nerds like me would have one, so use the most accurate digital meter you can find/borrow. Alternatively, use an ice-water slurry in a bucket, put the transducer in there, and calibrate to 32F. Don't forget to "soak" the transducer in the water for a good while to let it fully get to the temp of the water, it may be hot from riding to the lake in the sun.