The least accurate way to track mash temperature is to float the thermometer at the top.
The only reason there are "floating thermometers" is you can't read those if they don't float, but that's not a sound reason to follow suit as they are notoriously wrong wrt overall mash temperature unless one is recirculating - and then they'll likely read higher than the middle-to-bottom of the mash.
Which brings the next point: there are very few legitimately "water proof" temperature sensors, especially in the "wireless BBQ thermometer" world. Which is why most folks going electronic use through-wall fittings so only the business end of the sensor is exposed to liquids...
Cheers!