Very simply, the Ottoman Navy did not have any submarines fit to keep the IRN in check, their only boats were 2 Nordenfelts built in 1887 which had been laid up at Constantinople after their trials and never looked at again.
When German uboote arrived in the Med, some were sent to the Dardanelles and were very succesful ther, sinking BBs Majestic & Triumph (both by U-21) and many transports plying the Aegean. As a logical follow-up, the smaller UB type boats carried out operations in the Black Sea, to prevent it from becoming a Russian lake. Goeben & Breslau could not face all the threats the Russians were mounting by themselves and something had to be done both to support the Ottomans & to hinder Russian operations, hence UB-14 (von Heimburg), U-38 (Valentiner), U-21 (Hersing), U-33, UB-8, UB-7, UB-42, UB-44, UB-45, UB-46, UC-13, UC-15, UC-23, UC-74 and U-63 all carried out at least one patrol in the Black sea with various results.
If you have a chance to look at it, the most useful source on the subject is the 2-volume set "Der Krieg in der Türkischen gewässern". I don't have the names of all the COs at hand (maybe Michael can help you there).
Many of the sinking claims cannot be accurately verified due to the targets being mostrly small sailers or coastal steamers which don't show up in Lloyd's and the Russian records have been largely lost during the Revolution.
regards, Brian