Water & Electricity =?

My brother had a float house on a big lake (1200feet deep in the so. end) in Sandpoint Idaho. One weekend we went up there and here was the guy that worked for the marina standing in waist deep water cleaning a boat with an elec drill and the cord laying in the water. We freaked out but he said water doesn't conduct electricity it is the minerals in the water that does. This lake didn't have many minerals He has been doing this for years so he must have a good point.