Water temperature is useful to know for a range of reasons.
Do I want to go swimming?
Is there risk of ice forming overnight?
Do I need to shut down the reverse cycle heat pump?
Am I sailing in the Gulf Stream?
They sell fancy add-ons that connect to your chart plotter to tell you the water temperature, but these involve another hole in the hull and are rather costly.
Another option is to drop a thermometer over the side and record the water temperature, but this runs the risk of loosing your thermometer and watching it float away in your wake.
Yet another option is to pull up a pail of water and drop a thermometer in there, this is both safest and cheapest. The water is a fresh sample, and the thermometer can not be separated form the yacht. The only downside is it involves a lot of effort to raise a pail of water and keep the pail from splashing everyone near it. If you are trying to cross the Gulf Stream, you want to know the water temperature, but conditions may not permit this endeavor; leaving you wondering what the water temperature is. Bet you wish you had the fancy sensor connected to your chart plotter now!
There is another way that is both inexpensive and relatively accurate. Installing an Indoor/Outdoor Thermometer in the boat with the "Outdoor Lead" attached to the hull low in the bilge. By covering the sensor with foam insulation, it will be insulated from the interior air temperature and will pick up the temperature of the fiberglass hull.
This method will give you a relative idea of the water temperature that the boat is floating in since that will closely correlate to the hull temperature.
The indoor/outdoor thermometer will provide you with an easy way to watch the water temperature while underway without out complicated processes or expensive equipment.