Did American PT boats EVER score a single hit on an enemy ship with a torpedo during WW2?

A few, but not many. Part of the problem was that during the period where they were most active ('42-'43), U.S. torpedoes were garbage. Even if one managed to hit its target it was a tossup as to whether it would even explode. This was also the case for submarines and torpedo planes during that period. Ironically, P.T. boat crews were not highly trained in torpedo attack, which reflected a larger tendency in the U.S. Navy to emphasize gunnery, rather than torpedoes. Through the war, P.T. boats managed to get a few torpedo hits, one that stands out in my memory is a crippling hit on the light cruiser Abukuma in the Battle of Surigao Strait in 1944--and that was an accidental hit--PT-137 launched a torpedo at a destroyer, but the missile ran deep, passing under the destroyer and hitting Abukuma instead. It disabled her and she was later finished off by aircraft.

Where the P.T. boats excelled was not in torpedo attack, but in reconnaissance, rescue of sailors and aviators, and in harassment of enemy barge traffic in the South Pacific. The Japanese used a lot of shallow-draft barges for a lot of their movement of troops and supplies between various islands, these smaller vessels could run along the coastlines, close enough that they would be hard to see from the air, and just as difficult to see from larger U.S. warships. The PT boats could get in closer to the shore and strafe the barges with their automatic weapons; some of them were stripped of their torpedo tubes and fitted with extra guns, including occasionally 40 mm Bofors.

The romantic hype surrounding them was part of wartime propaganda to boost morale on the home front at a time before U.S. production had gone into full gear, and the issue in the South Pacific had not yet turned decidedly in favor of the U.S. And of course their post-war mythology was strengthened by one of their former skippers being elected to the Senate and later the Presidency.

From: https://www.historynet.com/the-truth-about-devil-b...

"Though few PTs sank major Japanese ships, they enjoyed more success in other operations, including reconnaissance and search and rescue. The boats often harassed and broke up Japanese barge traffic, earning the nickname “devil boats” among the enemy. During the 1944 Battle of Leyte Gulf, PTs provided vital intelligence on the movement of Japanese warships into Surigao Strait. And PT-137 scored a crippling torpedo hit on the light cruiser Abukuma."

So yes, at least one PT boat scored a hit on an enemy ship during WW2.