Trenches on the Web is the best WWI site on the web.
Subwar network has info on submarine warfare.
Includes an article on the Lusitania. Cunard archives at the university of Liverpool contains info on the Cunard line, including information and a picture of the Lusitania. Danny's Naval Home Page includes an article on the famous Q-ships. The War Times Journal contains online books on the first world war including one by Admiral Scheer on German Naval operations during the war. This page is about the American S-boats which began to be built in 1918.