316 Stainless Steel has good corrosion resistance but it can be damaged by crevice corrosion in salt water. The Stainless Steel versions of the HOBO loggers should not be deployed for more than 6 months (lifetime cumulative) in salt or brackish water or the housing will eventually fail and water will damage the electronics. How long the logger will last depends on many factors such as salinity, level of dissolved oxygen and temperature. Biofouling can also limit deployments in saltwater, because growths on the pressure sensor will change its calibration. The amount of biofouling that occurs depends on location and time of year.
For long-term deployments in salt water, we recommend either using the Titanium version of the HOBO Water Level Logger (U20-001-01-Ti or U20-001-04-Ti) or deploying the stainless steel versions in a sealed bladder. The Titanium version will not corrode in saltwater, but it is still susceptible to biofouling. In locations where biofouling is an issue, it may be possible to use an antifouling coating on the logger housing, but you must be sure that the coating will not adversely affect the environment. The coating must be applied such that it does not get on the sensor or block the hole to the sensor.
Deploying the logger in a sealed bladder filled with fresh water will protect the logger from corrosion and biofouling, but it does take up more space. The pressure outside the bladder will be transmitted to the pressure sensor inside the logger. The Aquapac Small Phone Case (aquapac.net) is a case that can be used as a protective bladder.