You can use the Advanced Search features in Mergent to create a list of companies within a particular industry. Use the NAICS lookup to find companies within the maritime industry.
“The North American Industry Classification System (NAICS) is the standard used by Federal statistical agencies in classifying business establishments for the purpose of collecting, analyzing, and publishing statistical data related to the U.S. business economy.” (http://www.census.gov/eos/www/naics)
Hint: try NAICS category 48: “Transportation and Warehousing”
Subcategory: 483: “Water Transportation”
Subcategory: 4872 “Scenic and Sightseeing Transportation, Water”