It depends on the sensor. The following is a list of the common Water Quality sensors and their resistance to salt water.

Temperature Probe – The stainless steel should do fine.

pH Sensor- It is a potentiometric sensor. Since the sample presents an impedance between the reference junction and sensing bulb, the saltwater might actually improve response time by lowering this impedance.

Dissolved Oxygen Probe – The Dissolved Oxygen Probe responds to the partial pressure of DO in the sample. Corrections for salinity would need to be made to the readings according to the solubility of oxygen in saltwater. This correction is documented in the DO sensor booklet.

Optical DO Probe – Corrections for salinity would need to be made to the readings according to the solubility of oxygen in saltwater. See the following link for correcting for salinity. While submersible, the entire Optical DO Probe is not intended for long term submersion.
How do I compensate for salinity when measuring dissolved oxygen?

Ion-Selective Electrodes: We do not recommend any ISE for long-term use in salt water. Specifics by probe are shown below.
Cl – This should work fine. It is a solid state sensor made to look for chloride.
NH4 – Not recommended for long term readings. The PVC membrane has a special coating that will be depleted over time. Chloride ions do not interfere with the response of this sensor.
NO3 – Not recommended for use. There will be interference from chloride ions.
CA – Not recommended for long term readings. The PVC membrane has a special coating that will be depleted over time.
K – Not recommended for long term readings. The PVC membrane has a special coating that will be depleted over time.

Conductivity Probe – The maximum reading is about 13 PPT (ocean water is 35 PPT) on high range, thus we don’t recommend it for salt water. The Salinity Sensor, which has a maximum of 50 PPT, is made specifically for salt water.