[line 1] the battered liners sank: on 4 February 1915, the Germans declared that from the 18th of that month, all the waters around Great Britain would be included within the war zone, and all “enemy merchant vessels” would be destroyed and their crews and passengers need not be saved. This might include vessels sailing under neutral flags, owing to British “misuse” of such flags. The most notorious such incident was the sinking of the passenger Lusitania on 7th May 1915, with the loss of many American as well as British civilian lives. Not long afterwards, Arabic was sunk, with 44 deaths, including two or three U.S. citizens.
[line 3] Walworth: part of the London borough of Southwark.
[line 18] pied craft: ships were painted black and white to break up their silhouettes, confusing the enemy as to their direction and speed, thus causing him to miscalculate the aim-off for his torpedoes, and so miss. Later in the war, other colours were used to blend in better with the sea.
[line 21] not even our rank: both “I” and “you” in the poem held temporary wartime commissions in the Royal Naval Volunteer Reserve. These lapsed when they were demobilised after the war, and they would no longer be entitled to use their ranks, although regulars and reservists with permanent commissions were permitted to do so. This rule was not always observed, as a military rank conveyed a certain cachet in those days; but to use one would have been seen as inappropriate for a grocer’s clerk, while for a bank manager to use one to which he was not entitled would bring social disgrace if he were found out.