Go to your local Lowes, Home Depot, etc. Buy an indoor/outdoor digital thermometer (not a wireless one). Set it on outdoor, put the probe in the water and wait 'till the temp stabilizes.
I bought mine for less than $10 and it has a probe that is 10' long.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).