Stanley, you are correct the Kinneret (Sea of Galilee) does not have currents and therefore there was no undertow.
On the particular day in question there was a "Hamzin" ... generally when we think of Hamzin we think of a very hot day...however, Hamzin is in fact a hot wind which over water can become a hot rapid wind.
The people in the water were faced with an unusual situation the wind shifted rapidly from prevailing westerly winds to prevailing easterly winds.
Meaning that the shifting winds pulled people out into deeper waters.

BTW, I doubt that alcohol or drugs were a factor. It is more likely that the boys didn't know how to swim and were unable to return to shore as they were carried out by the easterly winds.