Well, we are ignoring several things here - that since 1913 there has been a very influential German miulitary mission in the Ottoman Empire under Liman von Sanders, whose existence was aimed at making the empire pro-German and using that to exploit its strategic position. Also during early 1914, the Germans were attempting to bring about an Ottoman-Greek rapprochement - the details are in Albertini, and I have forgotten most of what I ever learnt, but it was a strategic goal. It tied into Austria-Hungary's courting of Bulgaria and was aimed at shackling Serbia, and replacing a Rumania increasingly viewed as unreliable. Added to that, Enver and ?Talat were pro-German and had already preliminarily committed the empire in August 1914.
The seizure of the dreadnoughts that would become Agincourt and Erin raised a mighty stink in Istanbul, whereas the arrival of the Goeben and the Breslau and their 'sale' to the Ottomans was an excellent propoganda coup for the Germans as they could be seen to be countering British seizure of ships by the provision of their own. Once Souchon was at Istanbul, the likelihood of the Ottoman Empire doing anything other than join the Central Powers becomes even smaller. He took virtual command of the joint fleet, and the bombardment of the Russian poets that brought Russia into the war was instigated by him without reference to all of the government in Istanbul.
A POD to have the Ottoman Empire remain neutral might be to have Troubridge engage Souchon - he may well lose his armoured cruisers, but if he can damage the Goeben she may never reach Istanbul, or might be caught by Milne as she limped away. This would be both a propoganda blow to the Germans, and remove an important impetus towards war. A neutral Ottoman Empire may then join the war on the ENTENTE side at a later date, depending on how things go elsewhere.
You might also want to kill the sultan. If you get rid of Mehmed in this period, Yusef Izzedin becomes sultan, and was a generally pro-Entente-minded man, believing increasingly that the Ottoman Empire's dalliance with Germany would lead to its ruin - hence his being 'suicided' in 1916 by Enver and co
Grey Wolf