Isle of Man Steam Packet Company England, Wales, Scotland and Ireland. Ports served included Liverpool, Silloth, Whitehaven, Holyhead, Ardrossan, Blackpool, Belfast and Dublin. Vessels and crews of the company were actively involved in both the First and Second World Wars. One vessel, , was attached to the British Grand Fleet and led the German High Seas Fleet into Scapa Flow in the Orkney Islands at the end of the First World War. Another vessel, "Viking", was converted to become a seaplane carrier, serving as . During the First World War, eleven out of a total fleet of fifteen Steam Packet ships were requisitioned by the Admiralty.
Naval warfare of World War I Fleet defended the port of Sulina throughout the second half of 1916, causing the sinking of one German submarine. Its minelayer also defended the Danube Delta from inland, leading to the sinking of one Austro-Hungarian Danube monitor. (See also Romanian Black Sea Fleet during World War I) When Bulgaria entered World War I in 1915, its navy consisted mainly of a French-built torpedo gunboat called "Nadezhda" and six torpedo boats. It mainly engaged in mine warfare actions in the Black Sea against the Russian Black Sea Fleet and allowed the Germans to station two U-boats at Varna, one of which