Diesel powered equipment has worked well over the years on small and large capacity forklifts involved in stevedoring operations.
The biggest issue with diesel is not carbon monoxide but the NOX, hydrocarbon & soot/smoke emissions. Today's engines are much cleaner running than those of the past - newer emission standards and catalytic converters.
The quality of the diesel fuel is something to watch out for. In some countries the quality is poor and injectors plug up quickly.
I have sold Linde diesels for use inside a closed warehouse - you could eat lunch by the tail pipe.
Not certain what you mean by a saltwater environment - dockside or driving thru it or inside or outside. But either way saltwater is not good for any lift trucks, gas, LPG, CNG, or battery powered. Corrosion is a major issue. Suggest a unit with DS or LPS safety rating features - like an enclosed alternators/starter, booted electrical terminals, etc. and make certain a good coating of diaelectric grease is applied on any and all electrical connections & connector plugs.
Diesel work best and are most efficient when they are worked hard, engine speeds are high (@1400-1600 rpm) - like the over the road tractor/trailers - the operators are always changin gear to maintain the engine "Rs". They loose efficiency in stop & go operators, where LPG is better, and at idle speeds - which cause fuel system issues.
Hope this helps.