Last year I had evacuated tube solar panels installed to heat my pool. This year I just started using a Raspberry Pi with two DS18B20 temperature sensors to measure the temperature of the pipes that carry the water to and from my panels. These are being logged to a database and I can then run reports from the database. The temperatures aren't exactly the temperature of the water in the pool, but seeing the delta between the two shows how much the water is being heated by the panels.
Here is an example of what the data looks like. My system normally shuts down from about 10pm to 5 am - but I kept in running overnight on May 23/24 to heat it up using the gas heater (the timer did shut it off at 10 but I turned it on an hour later - you can see a notch down in the chart that shows this). But you can clearly see when the pump shuts down for the night on May 22 and 24.
I will post more info regarding this on github if anyone is interested. I am also working on a floating solar/battery powered Raspberry Pi PC that will measure temps in my pool and perhaps pH and ORP (a proxy for chlorine?). But those probes, such as the ones from Atlas Scientific, are around $100 each all-in.