PT-59 (more on her later) is credited with sinking Imperial Japanese Navy (IJN) submarine I-3 on December 9, 1942 near Guadalcanal. PT-37 and PT-40 sank destroyer IJN Teruzuki on the night of 11–12 December, barely three months after the Japanese had put her into service. (Thanks to George Avery for his comment bringing this sinking to my attention.) They also damaged at least one cruiser, IJN Abukuma, during the Battle of Surigao Strait (1944). While trying to escape after the battle, Abukuma was bombed by B-24 Liberators and sunk, so PTs should get an assist there.
UPDATE (Nov 4, 2024): On 26 November 1944, IJN Kiyoshimo was crippled by US amy bombers and PT-223 finished her off with a torpedo. IJN Makigumo was also crippled and scuttled after hitting a mine while avoiding a PT boat, earning some credit for an assist. Thanks to Brandon Kai Chen Low for finding that information.
During the 1941–42 Philippines Campaign, PT boats were credited with sinking Japanese ships, but postwar assessment denied the claims. The greatest success they had was interdicting Japanese coastal shipping in the Solomon Islands and New Guinea areas. Since Allied airpower made it extremely hazardous to commit freight ships in those areas, Japanese reinforcement and supply was often done by Daihatsu landing craft (below).
Under cover of night, the Japanese would creep along the coast, hoping to avoid notice, but many PT boats carried radar, so the Americans were often able to surprise them and attack from ambush. Both sides up-armored and up-gunned their vessels. Daihatsus often carried 100 or more troops, and they would join in with their personal weapons as well, making these close-quarters fights quite violent. Some PT boats were converted to gunboats by removing the torpedo tubes and installing extra armament.
This image shows PT-59, at the time commanded by Lt. (jg.) and future president John F. Kennedy, after conversion to a gunboat. (Kennedy’s more-famous command, PT 109, had been sunk the month before.) Note the radar mast amidships, Bofors 40 mm anti-aircraft gun aft (another at the bow), .50-caliber machine guns behind shields (three per side), and absent torpedo tubes. I’ve read that PTs would sometimes carry 37 mm M3 anti-tank guns, secured to the bow, but more often they would use machine guns and 20-mm anti-aircraft cannon that were normally part of their armament, 40-mm Bofors guns (later also installed on many non-gunboat PTs), plus any other weapons the sailors could scrounge up.
This is an aircraft-type 37-mm cannon (not an anti-tank gun) mounted on the bow of a PT boat. It likely came from a P-39 fighter, which used this type of weapon.
A 20-mm anti-aircraft mount on a PT boat’s stern. Note depth charges left and right.
In Europe, Italian torpedo boats sank several freighters and damaged light cruiser HMS Manchester, which was scuttled (the commander was later court-martialed, it being thought she could have been saved). German torpedo boats (E-boats) claimed sinkings of over 100 freighters and a dozen destroyers, but their best-known victory was sinking two Landing Ship, Tanks (LSTs) during Exercise Tiger, killing 749 American servicemen.
Source: Bruce Brews (Quora)