They key is the number of dead, particularly the number of American dead. You can get an answer from nothing much changes (ships still has large loss of life) to it not that well remembered (ship is run a ground on some mud bank, no loss of civilian life). Just think of it this way, off the top of your head, can you list all ships sunk pre-April 1917 that had American civilians on board?
I only know offhand of three others. The
Arabic was sunk in August 1915, with 50 dead including two Americans. The
Sussex was sunk in March 1916, with 80 casualties including two Americans wounded. The
Laconia was sunk in Feb 1917 with three Americans killed (two of them women who happened to be friends of the First Lady).
The Lusitania was in a totally different class, with about 1100 dead including 128 Americans, and it was this more than anything which triggered President Wilson's string of notes to Berlin. $64,000 question is what happens if the
Lusitania's death toll had been similar to those later ones. Obviously even two dead Americans is, in principle, two too many, but one wonders if that would have had the same impact as the wholesale "massacre" on the
Lusitania. Would Wilson have reacted so strongly in such a case?