Do you mean for sharpening or steel grade type?
If the former I use these:
If the latter, I like several types of knife steel. One of my favorites for holding an edge and ease of resharpening:
D2 steel is an air hardening, high-carbon, high-chromium tool steel. D2 has high wear and abrasion resistant properties.
www.steelexpress.co.uk
For a more corrosive resistant, and edge retention steel
CPM 154 Steel is another option but it tends to be more expensive. Trade offs it takes more work to resharpen but holds the edge longer. In a Marine environment I would go with CPM 154.