Does it exist? Environmentally-friendly/safe fluorescent liquid to make sea crashes easier to find?
Thanks to those who commented and provided answer-worthy comments. Thanks!

What is already in existance today:

@Andy commented:


  For spacecraft splashdowns (certainly Gemini, maybe others) they used
  a green marker dye, but I don't know how long it lasted. That would be
  a good thing to start with as they must have tested how long the
  traces were visible when choosing it.


@RogerLipscombe commented:


  prestodye.com/sea-dye-markers says "30 to 40 minutes", depending on
  sea conditions. Apparently there's a standard: MIL-S-17980D.


And for the challenges:

@abelenky commented:


  Rough approximation: How many gallons of this stuff would you need for
  it to not dissipate in minutes? How much would that weigh? How often
  to airplanes crash in the sea? The cost and complexity just doesn't
  even come close to the benefit.