Greater then 50%
American submarines sank Japanese merchant ships.
Greater than 50%
Lusitania wasn't a German submarine, it was an American merchant ship that was sunk by German submarines.
Guerre de Course (Merchant Warfare).
To patrol (look for) merchant vessels in the sea lanes.
. . . submarine attacks upon merchant ships .
Submarine Force
We entered World War 1 because of a German submarine attack on U.S. merchant ships in 1917.
Caused individual merchant/supply ships to get together in convoys.
Submarine Force
The US Navy did not ever "blockage" Japan in the traditional sense, where one nation stations naval forces off another's coast, and declares that no ship from any nation may travel to the blockaded country. However, what it did do, was initiate a "de-facto" blockage of the Japanese Home Islands via unrestricted submarine warfare. That is, starting in 1942 after the declaration of war following the attack on Pearl Harbor, the US Navy's submarine force attempted to destroy the Japanese Merchant Marine, and thus effectively cripple the Japanese importation of goods, and virtually all trade with Japan was done on Japanese-flagged vessels. The US Navy was spectacularly successful in this regard. By mid-1944, the Japanese Merchant Marine was effectively extinct, and by 1945, even the coastal freighters which carried goods between Japanese cities had been exterminated.
That statement is literal, not figurative.