There are two excellent lists for those with mariner ancestors.
The Mariners List (seamen and their ships) offers a mailing list with many knowledgeable members able to offer advice and their own research.
The Ships List (passengers and emigration) http://www.theshipslist.com/
TheShipsList website, online since August 1999, will help you find your ancestors on ships’ passenger lists. It has immigration reports, newspaper records, shipwreck information, ship pictures, ship descriptions, shipping-line fleet lists and more; as well as hundreds of passenger lists to Canada, USA, Australia and even some for South Africa. Be sure to check the “special projects” . There are over 3,500 totally free access web-pages with new databases added regularly). To make best use of your visit, use the Navigation-bars (buttons and text) which are on the top of every page, to help you find your way around.