As well as an online collection of Seamen's Protection Certificates, Mystic Seaport Museum's website has research guides here:

[Seamen's Protection Certificate Register Database (via Wayback Machine)][2]

The [National Maritime Digital Library][3] might be useful to get more background information.

Also check [Cyndi's List][1] for the category: Occupations: Fishermen & Mariners.

  [1]: http://www.cyndislist.com/
  [2]: https://web.archive.org/web/20120407014807/http://library.mysticseaport.org:80/initiative/ProtectionIndex.cfm
  [3]: https://nmdl.org/