Age of Nelson.... Searchable database of men/ships in Trafalgar. I used this with Robertson as the surname and it returned a list; two of which were Shetland. This is an interesting resource for the particular time period. This was when the navy would call in at Shetland and press gang likely individuals into service. The site also has good sections on ships, articles and documents.
Naval History.... This is a substantial site that covers World War 1 and World War 2 campaigns, convoy escorts and casualties. This site is well worth a visit where you could spend some time. I was able to check out details of the August 1942 of the Malta Convoy. My uncle was first officer on the Ohio and it gives a brief account of the losses involved with the convoy.
Naval and Naval Social History.... Covers the period from 1790 to 1932 plus other items. A useful historical account of the period. Also includes, Engineers, Medical/Surgeons, Coast Guard and much more.
The Ships List.... An extensive range of options/links here. You can search by Passenger Lists, Fleet lists, Ship arrivals, Marriage at sea, and many others. I tried the Passenger List and entered Thomas Johnson Shetland in the Search Australia. It returns a list from the database displayed as a Google search would.