Most people use a probe sensor for this type of application. There’s no power unit in the probe sensor itself, it’s cabled to a separate device, which is what has the radio to communicate with smartthings. This allows you to keep the cost way down compared to something with a radio designed to actually operate underwater. Typically a ds18b20 sensor is used.
You can see examples of some of these that work with smartthings in the following thread.
Devices that work with DS18B20 Probe sensors that have working Edge Drivers (2023)