This site: https://en.wikipedia.org/wiki/Naval_operations_in_the_Dardanelles_Campaign has provided some more food for thought as to when German submarines arrived in the Eastern Med/Turkey. Will include excerpts from the site in quotes below:
One of these quotes indicates the presence of German subs in March 1915, which could mean they were deployed there before March 1915....
Keyes remained a firm supporter of naval action, and on 23 September submitted a further proposal to pass through the Dardanelles to de Robeck. De Robeck disliked the plan, but nonetheless passed it to the Admiralty. Risk to ships had increased since March, due to the presence of German submarines in the Mediterranean and the Sea of Marmara, where the British ships would be inviting targets if the plan succeeded. On the other hand, minesweeping was now better equipped and some of the ships had nets or mine bumpers which it was hoped would improve their chances against mines. The Ottoman Empire now had better supply routes from Germany whereas demands on the navy for more ships to support the attempt had to be added to continuing commitments of ships for the land action, and the ongoing campaign at Salonica attempting to support Serbia
The first French submarine to enter the Sea of Marmara was Turquoise. However, it was forced to turn back and, on 30 October, when attempting to pass back through the straits, ran aground beneath a fort and was captured intact. The crew of twenty-five were taken prisoner and documents detailing planned Allied operations were discovered. This included a scheduled rendezvous with HMS E20 on 6 November. The rendezvous was kept by the German U-boat U-14 which torpedoed and sank E20 killing all but nine of the crew. Turquoise was salvaged and incorporated (but not commissioned) into the Ottoman Navy as the Onbasi Müstecip, named after the gunner who had forced the French commander to surrender.
The Allied submarine campaign in the Sea of Marmara was the one significant success of the Gallipoli Campaign, forcing the Ottomans to abandon it as a transport route. Between April and December 1915, a total of nine British and four French submarines sank one battleship, one destroyer, five gunboats, eleven troop transports, forty-four supply ships, and 148 sailing vessels at a cost of eight Allied submarines which were sunk in the strait or in the Sea of Marmara.
In 1993, a coal mining operation revealed the wreck of the German submarine UB-46 near the Kemerburgaz coast. After carrying out missions in Black Sea, on its way back, UB-46 hit a mine near Karaburun and sank with all hands. It is now on display at Besiktas Naval Museum in Istanbul.
Also on 27 April, a kite-balloon ship had spotted an Ottoman transport ship moving near the Narrows. Queen Elizabeth, stationed off Gaba Tepe, had fired across the peninsula, at a range of over ten mi (8.7 nmi; 16 km), and sank the transport with her third shot. For much of the campaign, the Ottomans transported troops via rail, though other supplies continued to be transported by ship on the Sea of Marmara and Dardanelles.
It quickly became evident that the battle for Gallipoli would not be a swift or easy operation. At Helles, which was initially the main battlefield, a series of costly battles only managed to edge the front line closer to Krithia. Through the early battles, the Royal Navy continued to provide support via bombardments. However, in May three battleships were torpedoed: Goliath in Morto Bay on 12 May; Triumph off Anzac on 25 May; and Majestic off W Beach on 27 May. Goliath was sunk by the Ottoman torpedo boat Muâvenet-i Millîye while the other two were sunk by U-21.
Allied troops transports
An other important aspect of the allied naval operations was transporting safely the many thousands of soldiers to and from the Dardanelles over the Mediterranean Sea. The major threats were attacks by German and Austrian-Hungarian submarines and mines. The only major loss during the Dardanelles Campaign was the sinking of the HMT Royal Edward on 13 August 1915. The ship sailed from Alexandria, Egypt to Gallipoli with 1,367 officers and men onboard and was torpedoed by SM UB-14 near the Dodecanese. 935 lives were lost.