Dissolved Oxygen Sensors

The Gravity: Analog Dissolved Oxygen / DO Sensor Meter Kit (SEN0237) can technically be used in seawater, but it is not optimized for it and requires significant extra care compared to freshwater use.

if you do want to use it in seawater, be aware of that:

Salinity Compensation: The concentration of dissolved oxygen decreases as salinity increases. This analog sensor does not have a built-in conductivity/salinity sensor. To get an accurate reading in seawater, you must manually calculate a compensation factor in your Arduino code based on the known salinity (usually ~35 ppt for seawater)

Galvanic Interference: Because this is a Galvanic probe, it generates its own small voltage. High-salinity water is very conductive, which can sometimes lead to "ground loops" or signal noise if other sensors (like pH or EC) are in the same tank without proper isolation.

If you are planning a long-term marine or mariculture project, DFRobot actually offers a dedicated RS485 Fluorescence Dissolved Oxygen Sensor (Seawater version) designed specifically for high-salinity environments.