IIRC it was Adm Fisher who was defending the RNs role in the war who pointed out that without the RN, Germany could have carried out a Gulf of Riga - Operation Albion against the Cherbourg Peninsula for a coup de main against the French.
In 1915, the Germans were too busy pouring concrete and wallpapering their dugouts along the Western Front as the new 'frontier' to think of such operations against the French.
The Germans would only have limited capacity. In Friedman's, "Fighting the Great War at Sea" the author highlights RN studies into what the Germans could hyperthetically do. There was the spectre of all the shipping immobilised by the British blockade, sufficient, it was thought, for 250,000 troops.
In a test landing at Clacton in 1904, about 12,500 men, 2500 horses, 55 guns and 320 vehicles were landed from ten transports, six ships being cleared in an average of under 20 hours and four more averaging under 28 hours. It seemed that this underestimated what could be done in wartime, when safety precautions would be relaxed.
At Gallipoli, 29,000 men with seven days’ supplies had been disembarked in 12½ hours, but it helped enormously that the Mediterranean is tideless; on the other hand, the landing had been opposed fiercely.
In November 1915 the army’s Director of Military Operations (DMO) estimated that the Germans could assemble a force of 50,000 to 100,000 infantry at any time they were not mounting a major operation. The conference translated that as a ten-division threat. DMO added that he was not at all confident that the British would know about such an operation even at the point at which it embarked. In the instructions to be followed in the event of an invasion, the CID maximum estimate became a minimum enemy force.
A 1 January 1916 conference, chaired by Adm of the Fleet Sir A K Wilson, sought to determine the largest force the enemy might be expected:
(i) to transport to British shores and
(ii) succeed in landing before the operation was interrupted by the navy.
To avoid underestimating the threat, the conference assumed that enemy numbers would be limited only by transports. Recent experience (transport to France and, presumably, overseas) was that it took 6 GRT to transport each man, so the million tons available to the Germans equated to 170,000 men (a CID estimate of the largest force that could be embarked with artillery, etc., was 135,000).