The USGS has level gages at many lakes, and some of them include water temp. Several of my area lakes have them.
This is their interactive mapper, you can dig down to individual gages, then see what data each has available.
http://maps.waterdata.usgs.gov/mapper/index.htmlYou should be able to wind up at a page like the following for the lake I've been sailing on:
http://waterdata.usgs.gov/usa/nwis/uv?site_no=07229445Edit:
If the USGS doesn't have a gage on a particular lake, the Corps of Engineers may have it. They don't have a convenient site like the USGS though, and I'm not sure they bother with temp sensors. There are only a couple lakes in Oklahoma that I have to use a Corps page for level info, neither have temperature.