You and he were probably using entirely different generations of sonar.
could it have been sea mammals?
I had a Captain while doing my service in Finland he was convinced he had seen a UFO while aboard in Europe during the late 70s, he stood by it even when he was showed the spirit and claimed we must have copied it.
In the many years and countless thousands of miles of voyages that followed, I never heard a goddamned thing. Oh well.