A dye pack might be a bit smaller and lighter if it contained a buoyant carrier fluid impregnated with materials which were mildly radioactive. There are numerous airborne and orbital sensors which are extraordinarily proficient at seeing such things. I would worry that the horror and gnashing of teeth from the uninformed about thousands of airliners flying with such materials on board might torpedo the idea before it hit the water, but I believe that there are both safe and detectable materials which could be viable.