Thermocouple with a submersible wire probe. I've seen Chinese-made ones for about $25. Real accurate, cheap, easy to read, easy to calibrate.
I've a waterproof one made by Cooper. It has a hard-wired probe. It works great.
I'd rather have one with a type K replaceable probe.
Omega and Cooper sell fairly well on Ebay.. The handheld ones are nice.
The bench ones will monitor 8 to 12 probes at a time (neat!)
I've given up on regular thermometers since having found my thermocouple.