A diver was repairing something (what?) in the Ocean when he encountered a
Submarine. He banged on the hull and used morse code to tell the crew
that the ship has been mined and should ascend now or be blown up.
The ship complied and was captured by a surprised dive support boat that did
not knew what happened until the diver was pulled up.

I do think this story happened during WW1, it may have been later, too.
I need details like name of ship, location, name of diver, and
nationalities, as well as the exact story.

I think that this happened before scuba times, so it would be a metal
dive suit that requires a support boat.