Forgot to mention: The location of the wreck does match a Bulgarian defensive minefield laid earlier in 1916. Key to the identification is that the wreck is fitted with a Bethlehem 37mm gun. UB 8 when under Bulgarian control as Podvodnik No. 18 is known to have been retrofitted with a Schneider & Cie 47mm gun, so can be excluded. The third UBI in the Black Sea was UB 14, which we now know was scuttled off Sevastopol.

Best wishes,
Michael