I keep seeing reference to dye bags. I don't think those will be of much use. Back in my P-3 days, we had Fluorescein dye in the sonobuoys. They were visible, but not that great, and they dispersed. I suspect the oil slick is more visible - especially to radar because it changes the surface reflectivity by dampening shorter wavelength surface waves and ripples. Also, you have to reliably disperse the dye, and yet keep it from leaking out with the aircraft traveling 100's of knots through heavy precipitation.
If it's really necessary to disperse something, how about a lot of little, very light floating radar reflectors - essentially aluminum foil that springs into a 3D shape in water? They could be tuned to common search radar wavelengths - all this requires is that the little antennae be the right length. Military search radar, like on the P-3 or P-8 should have no trouble finding this, day or night. Done right, they could also show up on orbiting SAR (radar) satellite imagery.