I came came upon this formula for getting an approximate water temperature. I have not had a chance to prove or dissprove it but it may get you close.
Take the high temperatures for 7 days and add them together and then divide by 7 to get the average. Take the lows for those same 7-days, add them together and then divide by seven for that average. Take the two averages and add them together and then divide that by 2. The final answer should be close to the surface water temperature. UNLESS you got 90 mph winds blowing.
I can't leave now; They fixen to turn on.