In my opinion not breakable is most important. Cheap is nice. Easy to read is really nice. I like
this one at Zoro.com. It only reads in Fahrenheit with a limited range of 25-125°F. The dial face is even plastic - not glass.
Whatever you buy, check your thermometer in a foam cup full of ice and water. If both ice and water are present the temperature in the cup will be 32°F (0°C). If your thermometer is off a couple degrees in either direction, close enough. If more accuracy is important to you (say for a hatch journal), measure
and record the offset once per year. For instance, if you measure ice water at 35°F, your offset is +3 degrees. Your thermometer should always be off in the same direction, the same amount. If you shock it really hard, say by leaving it on the top of the car as you drive down the road
, you should probably re-calibrate.
If you are looking for relative measurements - is this part of the pond/stream warmer than that part of the pond/stream - it will be fine without even checking it.
Tom
(if you couldn't tell, I'm a bit of a geek with stuff like this. That's the consequence of 40 years of being a chemist.)