I've read about them, and seem to recall they were planned for use in the Japanese Home Islands Defense.
Not sure if any were present, or used in Okinawa, but given the heavy losses of landing craft and landing ships, I guess it is possible some were present and effective.
I doubt any would have survived the blast from their pole tipped mines, but if they were lucky enough to have done that, they would probably have been crushed by the sinking vessel falling on top of them.
There is a book called something like German and Japanese Secret Weapons of WWII. It is very interesting, and the first place I saw references to these.
Suicide boats were used by the Japanese at Okinawa, apparently.
They also used suicide minisubs at Pearl Harbor, and in Australia. Most at Pearl were unsuccessful, but it believed that one minisub crew did launch a torpedo at one of the battleships in the harbor, during that attack. There was a documentary on either the History Channel, or Discovery, about that, and they found the sunken minisub in one of the estuaries feeding into the harbor.
Supposedly, it was found, and used for testing during the war, by the US Navy.
The attack in Australia achieved a few successes as well.
Similarly, the Italians and the British used minisubs to good effect on a number of occasions, against heavy warships in port.
