He found the wreck.
Today, with all the modern navigation instruments, people have little understanding how difficult it was to locate and return to shipwrecks many miles from shore, when You only had a sextant and a compass to define your position.
After many dives, the wreck lies in 55m depth, only few valuables were recovered. This included a fair amount of a precious stone called Rubellite.
https://bengems.com/en/stones/rubellite/
The largest crystal was something like 2600 carats.
I made a few dives myself.
There were some large Rock Crystals as you mentioned, but they were damaged by the fire. The British cruiser had lobbed a few large caliber grenades into the castle of the Wakama, to speed up the sinking.
In 1940, Rock Crystal was used to produce the highest quality camera lenses.
D. got excellent documentation, but the ship is surrounded by a lot of mystery. In the end, a large salvage vessel came from Germany and dismantled the wreck.
As far as I know, no diamonds were found.
Could it be that the information about the box of precious stones Rubellite was distorted to become diamonds?
D.'s next target was a shipwreck from the time of WW1, with something like 11 tons of gold on board.