How big is the pipe that you are looking to monitor?
What I am thinking is a probe set into a sleeve-mount around the pipe, sealed with RTV 732 sealant, allowed to cure before being fitted to the pipe.
Bear in mind you will need to drill into the pipe to give true temperature reading.
I don't think this is necessary unless the temperatures change very quickly. Temperature gradients are linear; if you lag around the probe with enough insulation, it will accurately read the internal water temp even if the probe is outside the pipe.
