Come to think of it, if you need a marker that's detectable at low concentrations and remotely (for example, from a maritime patrol aircraft), isn't the answer a radioactive substance? It's been done for various purposes.
Vague memories of my father doing this a few times. IIRC snag is to have something with a unique signature which is both detectable and which can give a concentration (for tracing currents) a fairly short half life is required. The stuff (mostly a gold isotope) was dissolved and in the sea within 24hrs after it came out of the reactor.