Waterproofing products
PCB encapsulation, potting or coating is usually against short time water contact, most potty will absorb moisture over time and ultimately get to the PCB over time if immersed. 

Water will also be absorbed through the cables and you need to use cables that are specifically designed to be under water. 

The correct way is to use a waterproof enclosure designed for it and that has seals.

For measuring the water temperature, you may find aluminium or stainless steel enclosure and you simply need to connect the probe to the wall of the enclosure.