Hi, In situations like this, I would always use a Stainless steel cabinet with Rittal type SK Stainless steel Rain hoods over SK fan / filter units. You will need to work out the airflow for the enclosure based on the heat rejection value from the Variable speed drive you are using.
The rain hoods slide up to gain access to the filter unit for maintenance, this should also stop any salt water spray from entering the enclosure.
Regards
Dave