The UK & Ireland Genealogical Information Service 
 aka GENUKI . A wide ranging resource with useful notes on a variety of archives. 

Tracing British seamen and master mariners by Bob Sanders in Wales.
An excellent resource with specific information on master mariners of Cardiff.

Immigrant Ship Transcribers Guild building an important resource.

  Caernarfonshire WalesGenWeb  maintained by Jayne Primmer. 

These sites all carry links to help with your research.
Good Luck!