When I teach this I use the illusion that is done in physics when you put a stick in a fish bowl or shine a torch into water, with the stick or light appearing to bend.
Its based on the same physics, conductivity of the medium to the propagation of electromagnetic radiation (radio or light waves)
The conductivity of the water is better than land, leads to greater reception range over water. The radiated signal from the NDB is not attenuated as much over water.
It is the apparent change in transmission medium that leads to the refraction.
If you were a chopper pilot going from a land base to a rig that has an NDB you encounter the same refraction issues when going from land to over water.
Z