Allied submarines operated in the Dardanelles even before the attack on its defences by surface ships in February 1915 and the amphibious landing on 25 April. HMS B11 sank the old Ottoman battleship Mesudiye in December 1914. The French boat Saphir was lost on 15 January 1915 after either running aground or hitting a mine.
Once the armies were ashore the submarines had to enter the Sea of Marmara in order to prevent Ottoman supplies from reaching the Gallipoli. Getting through the Straits in order to attack enemy shipping in the Sea of Marmara was very difficult. The boats had to pass through minefields and steel-wire anti-submarine nets without any metal blades to cut through the nets or the mine cables. They also had to avoid shore batteries and patrol boats and get through a 10 fathom deep stratum of fresh water that often made it very hard to control a submarine.
Most of the British submarines operating in the Dardanelles were of E class boats. In April the first British submarine to try to get into the Sea of Marmara, HMS E15, ran aground and had to be destryored by a British boat expedition in order to prevent her being captured. The Australian HMAS AE2 was more successful in getting through the Straits, but was sunk by an Ottoman torpedo boat: all her crew survived to be taken prisoner. The patrols of HMS E14 in April and May 1915 and E11 in May and June were, however, highly successful.
The Ottomans were able to send some supplies to their troops by land as well as the larger quantities carried by water. Submarines, however, managed to attack the land as well as the sea routes. On 17 July HMS E7 blocked the railway from Istanbul near Kava Burnu at the entrance to the Gulf of Izmid by bombarding a cutting and then shelled a troop train that had been forced to turn back from the obstruction. E7 subsequently attacked another train and a bridge without doing much damage, but she had done enough to show that the railway was vulnerable there.
E7’s captain, Lieutenant-Commander Archibald Cochrane, was awarded the Distinguished service Order. The citation, from naval-history.net [the other citations below are from the same source], stated that it was awarded for:
For services in a submarine in the Sea of Marmora, where he did great damage to enemy shipping, and after blocking the railway line near Kava Burnu by bombarding it from the sea, shelled a troop train and blew up three ammunition cars attached to it.
E11 also attacked the railways during her second patrol, as well as sinking several merchant ships and the old battleship Barbaros Hayreddin. her captain, Commander Martin Nasmith VC, was not satisfied with the results of bombardments of the railways, so on the night on 20-21 August her First Lieutenant, Guy D’Oyly Hughes, went ashore to sabotage the track. The citation for his DSO stated that it was awarded:
For his services on the 21st August, 1915, when he voluntarily swam to the shore alone from a submarine and blew up a low brickwork support to the Ismid railway line, in spite of the presence of an armed guard within 150 yards of him. After a running fight of about a mile, he dived into the sea, and was finally pulled on board the submarine utterly exhausted, having had to swim nearly a mile in his clothes.
The Allied policy was to have two submarines in the Sea of Marmara at all times. According to the British Official History, they were able to make the enemy’s supply ‘so restricted and precarious that the maintenance of the Turkish army in Gallipoli was a matter of grave concern.’ However, E11’s second patrol ended on 8 September and E7, her replacement, was lost after being caught in submarine nets on 4 September. After battling 12 hours to free her Cochrane was forced to scuttle her after she was badly damaged by a depth charge. He and all his crew were captured, leaving only E2 in the Sea of Marmara. She put Lieutenant H. V. Lyon ashore on 8 September with the intention that he should repeat D’Oyly Hughes’s feat, but nothing more was heard from him.
E2’s cruise ended in the middle of September. She was replaced by E12, which had a 4 inch deck gun rather than the 12 pounders fitted to her sisters. She was later joined by H1, the first of a new class of boats about half the size of the E class. The H class were built in the USA but fitted with their armament in Canada in an attempt to evade neutrality regulations.
These two boats were joined on 22 September by the Turquoise, the first French submarine to reach the Sea of Marmara, and by E20 the next day. Three days later E12 headed back to base after a 40 day patrol, the longest yet carried out in the Dardanelles: she was damaged after being caught in the nets and then attacked by six enemy patrol ships as well as shore batteries, but made it back. H1 completed her 29 day patrol on 31 October.
The captains of E12 and H1 were awarded the DSO. The citations read that:
Lieutenant-Commander Kenneth Mervyn Bruce, R.N. For his services in command of a Submarine in the Sea of Marmora, where he made a prolonged cruise, during the course of which he inflicted much damage on enemy shipping, and engaged and put to flight by gun fire a Turkish gunboat and a destroyer, and subsequently displayed much coolness, and resource in extricating his boat from a difficult position.
Lieutenant Wilfrid Bayley Pirie, R.N. For his services in command of a Submarine in the Sea of Marmora, where he inflicted much damage on enemy shipping, and co-operated with Lieutenant-Commander-Bruce in the chase of a Turkish gunboat.
The French submarines operating in the Dardanelles, which were older and smaller than the British E class boats, had found it hard to get through the Straits. The Joule struck a mine on 1 May, while the Mariotte was forced to surface near the Chanak batteries on 27 July after being trapped by the net defences. She came under artillery and was lost, with her crew being captured.
Nothing was heard from the Turquoise from 26 October until a German telegram claiming that she had been sunk on 30 October by gunfire and her crew captured. In fact, she had been stranded and captured intact. Her confidential papers, including the details of a planned rendezvous with E20 were not destroyed. When E20 arrived at the rendezvous on 5 November the submerged German U-boat UB14 was waiting for her. UB14 fired a torpedo from a range of 550 yards, sinking the British boat. Only nine of her crew survived.
E11, which had returned to the Sea of Marmara on 6 November, was now the only Allied submarine there. Her third patrol lasted until 23 December, during which time she sank more steamers and the destroyer Yarhissar and bombarded the railways. Her three patrols totalled 97 days, and she was credited with sinking or rendering useless a battleship, a destroyer, five large and six small steamers and five large and 30 small sailing vessels. Nasmith was promoted to Captain after only a year as a Commander. E2 joined E11 on 10 December, but the Gallipoli Campaign was coming to an end.
The British claimed that a battleship, an old coastal defence ship, a destroyer, five gunboats, 11 transports, 44 steamers and 148 smaller vessels were sunk by submarines in the Dardanelles for the loss of four British and four French submarines. The German official history says 25 steamers of 26,000 tons and 3,000 tons of small craft were destroyed plus 10 steamers of 27,000 tons damaged and put out of action for the duration of the campaign. Paul Halpern suggests that the difference may be partly explained by some ships being beached and later repaired and refloated.
 J. S. Corbett, H. Newbolt, Naval Operations, 5 vols. (London: HMSO, 1938). vol. ii, p. 140; R. Gray, Conway’s All the World’s Fighting Ships, 1906-1921 (London: Conway Maritime Press, 1985), p. 208.
 A. J. Marder, From the Dreadnought to Scapa Flow; the Royal Navy in the Fisher Era, 1904-1919, 5 vols. (London: Oxford University Press, 1961-70). vol. ii, p. 312.
 Corbett, Newbolt, Naval. vol. iii, p. 77.
 Ibid. vol. iii, p. 119.
 Ibid. vol. iii, pp. 118-19.
 Ibid. vol. iii, p. 161; Gray, Conway’s 1906-1921, pp. 88, 92.
 Corbett, Newbolt, Naval. vol. iii, pp. 177-79.
 Ibid. vol. iii, p. 78; Gray, Conway’s 1906-1921, pp. 209-10.
 Corbett, Newbolt, Naval. vol. iii, p. 179.
 Ibid. vol. iii, pp. 205-6.
 Ibid. vol. iii, pp. 206, 217-18, footnote 2 on p. 218
 P. G. Halpern, A Naval History of World War I (London: UCL Press, 1994), p. 119.