I use a $15+- one from Orvis.com. Or get at any good fly shop. It's small and pretty accurate. I've owned a number of fishing thermometers over the years (including electronic depth-sensitive ones) and these little ones seem to be the best. They will break if you drop them, however. But they are only $15+-. Beware, though, that I've had plenty of them be at least a little bit off in their readings. Most seem to be consisant, but a few degrees high or low (my current one is 1.5 degrees to high.) So what I do is compare it's reading (at home) to any other thermometers (fishing or otherwise) I have around the house, such as my home's master gas heating thermometer that's on my wall, and garden thermometers I use for the yard. But you might not want to go to all that trouble. Or just compare it to a buddy's thermometer, once, while on a stream.