Simplest ideas are usually the quickest to deploy, and often the most reliable.
Place one or more blocks of bog-standard marker dye under seats or behind panels.
The passenger compartment invariably breaks up on striking the sea surface or is crushed by depth pressure, thus exposing the dye to water and starting a trail in the sea current.
Any airline could do this right away as a voluntary measure. Very little design required, no airframe changes, no type approval or certification.
The extra weight? Probably no more than a couple of bottles of booze.