I was just advised by DAN in their revision of their course materials for Hazardous Marine Life Injuries, that the tried old "tried and true" of vinegar was worthless. The idea had been that it wold effect and neutralize the nematocysts and at the very least help with the pain.

Current recommendations include using H20 110F if unavailable disposable hot packs, or boil water in a pan on the manifold of the boats engine. Of course if you are on a live aboard hot water will be available