Also, you don't want to measure the water temperature too close to shore due to sunlight heating rocks etc. so to do the job right, you need a buoy to support the sensor string, with a battery + solar panels to keep it charged and a wireless link back to shore for the data.
Placing sensor buoys without following the formal procedures of your local navigation authority is likely to result in their removal and possible legal action, and getting appropriate permissions as a private individual may be difficult or expensive so this sort of project is best done in collaboration with a nearby university or technical college.
The only exception would be if there is an existing open framework pier or a pontoon that you could get permission from the owner to let you hang the sensor string from in deep enough water.