Seems plausible. "Tales of the notorious kamikaze pilots, who crashed their explosives-laden aircraft into enemy ships, have survived long beyond their deployment in World War II. But, less commonly known are the kamikaze motorboats – the suicide attack vessels that took to the seas during this same period, equipped with explosives that detonated by switch or impact. Half a mile off the coast of Japan, a diver has discovered what appears to be the remains of Shinyo kamikaze boats lying on the seafloor 32 meters (105ft) deep. While these boats were vulnerable by nature and often never even reached their targets, experts suspect the newly-found remains came from vessels that survived the war, but were deliberately sunk after Japan’s surrender. Pieces of the suspected Shinyo suicide motorboat were discovered by 79-year-old Hiroyuki Arakawa during a dive roughly 1 kilometer (.6 miles) northwest of the Hasama fishing port in Tateyama, according to The Asahi Shimbun. There, the diver stumbled upon what’s thought to be a 1-meter-long engine, a 30-centimeter-wide propeller, and what could even be the remains of explosives. It’s been several decades since the war came to an end, leaving the artifacts to decay for more than 70 years beneath the ocean’s surface." WWII kamikaze boat remains found underwater off Japan | Daily Mail Online
John Ray Skates reported that there were thousands of these ready for using against the invasion of the Home Islands.
Over 6000 for the navy, and 3000 Maru-ni for the army, according to this- Shin'yō-class suicide motorboat - Wikipedia
There's some additional information at: OLYMPIC VS KETSU-GO and Pacific War, Operation Downfall The former mentions 9,000 boats. The latter mentions 500 mini subs under construction. Manned torpedoes are also mentioned on both sites. Could either of these be possibilities for the find above?
There was considerable discussion of them not too long ago over on j-aircraft. It had come to light at the time that one or more of them may actually have succeed in scoring torpedo hit(s) at Pearl Harbor. There was considerable back and forth in this discussion. Here are links for two ways to access it: "THE FIFTH PEARL HARBOR MIDGET SUBMARINE": THREAD NO.1 The Fifth Pearl Harbor Midget Submarine Here are some additional links: Type A Kō-hyōteki-class submarine - Wikipedia Midget submarine - Wikipedia The Midget Subs That Beat the Planes to Pearl Harbor https://gizmodo.com/watch-marine-archaeologists-explore-sunken-japanese-min-1789774035 Japanese Mini Submarines at Pearl Harbor | Office of National Marine Sanctuaries *** edit for *** Some more threads over on jaircraft MIDGET SUBMARINES & THEIR "HA" NUMBERS! NEW: "THE FIFTH PEARL HARBOR MIDGET SUBMARINE": THREAD NO.2
Speaking of suicide boats specifically, Giangreco says "Japan's surrender froze the production of these suicide craft at 2,412 for Operation Ketsu-Go, but measuring an average of only eighteen feet long and powered by inefficient, but readily available, automobile engines, the Japanese were on their way to easily tripling this number by the time Olympic was launched." Hell to Pay, p. 131. I go to Giangreco for hard numbers more often than any other source.
Skates states "The navy estimated that by the time the invasion took place, it could have as many as two thousand of these small, fast craft, designed to out at night..." The Invasion of Japan: Alternative to the Bomb", p. 111. He doesn't mention the Army numbers. Allen and Polmar don't mention the small boats at all, but have numbers for other craft. Codename Downfall: The Secret Plan to Invade Japan. Frank gives a number of 6,200 for the Imperial Army. Downfall, The End of the Imperial Japanese Empire. p. 184
