Are any countries installing remote sensors on commercial airliners to monitor oil spills, algal blooms,or other undesirable processes/features in coastal waters or inland seas (e.g. Baltic Sea, Mediterranean Sea, etc.)
Not to my knowledge. There would be a fairly high cost for the airline (both in installing the equipment and maintenance/running costs) plus the extra weight would mean they would have a lower passenger and cargo capacity. Airlines would therefore be very reluctant to install such equipment.
I also don't really see the benefit, the view from aircraft is not necessarily better than from satellite, particularly as the geo location and calibration would be challenging.