Can you water ski off of a jet ski?

Yes, the more powerful the jet ski the better. A pole attachement of the back will probably make it easier. You can waterski behind pretty much anything - the mythbusters did it behind an rowing team.

In most States, the jet ski must be rated for three people and you must have an observer to notify the driver if the skier gets in trouble. In some States, mirrors can substitute for the observer, but this is extremely dangerous. The driver should be concentrating on driving and someone should be watching the skier at all times so they can alert the driver if the skier is in distress. The skier counts as an occupant.