Q Ships (picture left about Q ships).  These ships were decoy ships - merchant ships armed with disguised heavy guns.  They were designed to fool U-Boats into attacking well-defended targets.  The immediate effect of Q ships was a reason why in 1915, the U-Boats stopped warning ships that they were about to be attacked.

Mines (picture left about mines) destroyed more U-Boats than any other weapon.  They were particularly effective in preventing U-Boats from using the English Channel and sailing into British Ports.

Convoys (picture left showing convoy):  From mid-1917 almost all merchant ships traveled in convoys.  British and USA warships escorted merchant ships in close formation.  Allied shipping losses fell by about 20% when convoy system was introduced in the mid-1917.  Depth Charges were even more effective when used together with the convoy system.

Depth Charges (picture left showing mines) these were bombs set to go off underwater at certain depths.  These were introduced in 1916 and proved second only to mines as a weapon against the U-Boats.

Long-Range aircraft (picture left showing aircraft): By the end of the war, aircraft technology had developed so much that aircraft could protect convoys.