Water Safety Instructor is a big, fancy job title for a person who teaches other people how to swim and be safe in the water. This usually involves teaching swimming lessons, often to children under the age of ten. This is an often annoying process that requires a person to sing songs, play games (like Red Light Green Light) and otherwise make an ass of oneself.
WSIs are trained by the American Red Cross and are required to be authorized in the region where they teach. The training is pretty arduous, and requires instructor candidates to be proficient in all the major strokes, as well as diving and teaching methods.
All told, being a WSI can be a greatly fulfilling experience. Where else do you get to see a kid get a huge smile on his face because he finally mastered the backstroke?