Halycon's Breech [PLEASE HELP] D12 Trench Discovery

[Dojii 18]

I'm trying to do this discovery I found on the atlas wiki called "Halycon's Breech".  Wiki says it's in the Trench.  If anyone finds this please let me know it's the last discovery I need in D12.  I spent 3 hours underwater with my Spyglass to no avail.

 

Breech is the butt of a cannon, before the shaft.  Idk....

[Sergey 4]

A similar situation in all the Golden Age Ruins. There should be 10 discoveries under water, but only 9 can be found.

Please tell me how to find these discoveries (they are all identical)?
C6 - Wayward Wall
D12 - The Waterway
F8 - Ollion's Wall
G4 - Arvendon Hold
H6 - Maetrine Stronghold
J11 - Halcyon's Breech
L3 - Citadel Wall
M12 - Deepend Hold
O7 - Captain Falk's Wall