I bought a stream thermometer:

[AndyS]

.......at my local sporting goods for $9.00.

I bought it to further enhance my logbooks by keeping water temps when I go fishing. Not only that I will be tracking the water temps in the lower Raritan river this summer because I am still amazed about the 5" brown trout we got in the seine net in South Bound Brook. Water temp today in Manville was a cool 64*
I think a good investment all around, one thing I really haven't looked into much is water temp.