Jim, Adam: I would be interested to hear if you have considering using the non-contact toroidal conductivity sensors from Sensorex for water monitoring since you have concerns about biofouling and you are in a saltwater environment. The inital cost of the toroidal sensor is high in comparison to the contact type sensors , but over the life of the sensor the initial cost I believe would be offset by the quality of the data and the large reduction in labor for sensor maintenance. https://sensorex.com/product/tcs3020-toroidal-inductive-conductivity-sensor/
I have several years using these sensors in industrial applications that required the biofouling resistance. I also helped deploy torodial sensors aboard a sailing vessel that was monitoring both freshwater and sea water. I am interested to hear your feedback. What is your expected range of conductivity measurements.