I'm somewhat of an veteran on this, having been burned by two $40 digital waterproof (but not heatproof past 122f, I found out) thermos. After being told that I wast stupid for not "knowing" that the sheathing on those thermometers was only rated to 122f, even though none of the literature said so, and subsequently writing a scathing letter to the company, they gave me a refund for both failed thermometers. When I spoke to the company that made the failed ones, they said that those were not good for beer brewing, and recommended a "Type K" thermocouple.
So with my refund $$ from the previous failed ones, I purchased this indicator
...and a type-K probe
(scroll down to "PTFE/FEP Tip Probes", I bought model #113-372-T):
I drilled a small hole in my mash cooler lid to feed the probe into the mash, and have been very happy with it. You can find the indicator and probe for less $$ on eBay, but after all those failed thermometers, I wanted the peace of mind of a supplier-backed warranty.