Thank you Ian and Bill.
The injector nozzles on all 4 cylinders were replaced with new 125 hours ago, as were the racor filters and the fuel filter. Today the racor filters and their "bubble" look pristine.
I am pretty sure what we see in the water is fuel. Since the water here is so clean and still, this afternoon I ran an experiment as suggested by Gary of Adagio: take diesel-absorbing pads and check if they absorb the slick. They do.
Maybe we are down to low cylinder compression in one or more of them, for whatever reason, or maybe we have a bad injector.