Marine Instrumentation

Emerson's DeltaV is PAC (Process Automation Controller) with DCS server-based functionality.

An Emerson document http://emersonkc.cafe24.com/main/images/DeltaV.pdf
describes some of the considerations when putting one into marine service.

Field Instruments have materials of construction that take into account for salt water corrosion.

In addition to what was previously said, the robustness of construction is taken into account. This is due to the environmental conditions that the device is going to be in (i.e. exposure to water, salt, waves, etc...). Some manufacturers make devices specifically for marine service.