funny, but I have actually seen someone sail a bathtub - no joke.
i've also seen someone surf on a upturned table, ski behind a rowboat (mythbusters) and some (crazy germans) water ski behind a hobie cat.
So, sailing a large rock, maybe not, but a kitchen sink is possible.
yeah i used to sail hobie 16's at forster but got over it coz the club is full of old men.
regattas still are an excellent excuse to get wasted though...