Hi LizzieW - I found the crew agreement at the Maritime History Archive in Newfoundland. They have most, but not all, of the crew agreements. I was actually tracing my g-grandfather who was also a sailor/fisherman.
It was a bit of a stroke of luck that I found him at all, because while captains were registered, ordinary seamen were not. You need to know the ship, then you can look up the crew agreement if it still exists. I found my g-grandfather because my grandfather recalled his father used to visit a Captain Enon. I gambled that maybe they had sailed together. I looked up the captain, looked up what ships he served on, and then looked up the crew agreements. Got lucky on the first ship.
Crew agreements exist for foreign going vessels and coasting vessels, but fishing vessels under a certain size didn't need to be registered. Whether there is any record of your g-grandfather will depend on what kinds of ships he worked on. And, you need a ship or a captain's name. If you haven't seen it already, the Crew List Index Project has a good discussion on how to find seafarers:
http://www.crewlist.org.uk/findingonindexes.htmlJennifer.