I haven't used/found one yet.
That said, for what its worth/helpful -
The experimental path I am thinking of going down is Ion Selective Electrodes. Eg. : https://www.amazon.com/INSTRUMENTATIONHUB-ISE-CA-1-Calcium-Selective-Electrode/dp/B0CNYJCX19
I was thinking of using the neptune ASM module with this .. unsure what the calibration/accuracy/durability/saltwater compatibility is of the electrode, however, ISEs may be a solution for continuous monitoring of water chemistry as they are available for an increasing number of elements .. would be amazing if these were implemented in a MEMS chip.
The current pH and salinity probes widely available and sold by neptune work on the same principle.