In that case ,if you plan on using thermal detection for differentiating a body of water against a thermally contrasting background, you could easily setup an inexpensive FLIR infrared camera on the drone and have it controlled by companion computer.
Then you setup a search pattern with mission planner, http://ardupilot.org/planner/docs/common-planning-a-mission-with-waypoints-and-events.html#auto-grid and do a conditional landing on thermal trigger using dronekit http://python.dronekit.io/about/overview.html.
Here is an example of this setup, using a simple camera system and detecting a Red Ballon
QuadCopter Object Tracking on a budget