Before the start of World War I there was a large arms race between Britain and Germany and although Britain didn’t come out with the largest army – they had by far the largest navy in the world. This raises the question, if British had such a large navy why didn’t they use it to evade the German trench lines and attack unsuspectingly from behind? The trenches couldn’t possibly exist in the oceans! Britain could’ve easily used its navy to either surround or outflank the German trenches.
Britain didn't use their navy because ships don't work on land. You need boots on the ground to exert control. They could have bombed the few coastal cities and fortifications, but it wouldn't have achieved much. It would still be necessary to break the enemy line and posses their territory which the ships couldn't do. All that could be achieved is the capability to bomb the crap out of a small section of trenches slightly more than land based artillery alone could manage.
The small advantages they had weren't completely without risk either. Just because a fleet is larger doesn't make it invincible there is always the risk the smaller feat can destroy the larger one. And in addition to enemy naval ships there are still mines and coastal fortresses to worry about. On mines specifically from Wikipedia
The total number of mines laid in the North Sea, the British East Coast, Straits of Dover, and Heligoland Bight is estimated at 190,000 and the total number during the whole of WWI was 235,000 sea mines.
The potential gain just wasn't there for the navy to be that useful in breaking trench warfare. Amphibious assault just wasn't a very practical approach until WWII technology was introduced.
The British tried a large scale amphibious landing during WW1 (1915) at Galipoli. It was a complete disaster. The experience went bad enough for them that they gave up on the idea for the next several decades until better equipment and air power made the idea feasible during WW2.
Operation Shingle during WW2 is an example of this idea of large scale amphibious flanking. The idea was to land in Italy to outflank the German Winter Line. This operation was marginally successful but came with massive risks and almost failed abysmally. Add to this that British forces would not have the numerical advantage that the Allies would have over the Italian and German forces in WW2 and little armour would be available to push home any advantages gained before the Germans responded.
The landing at Cape Helles and the Battle of Tanga show two different ways during WW1 that these assaults (which are both risky by nature and during WW1 still in their tactical infancy) can go horrible wrong.
While the British did have dominance of the North Sea, the German High Seas Fleet had not been defeated at Jutland and doubtless would have contested such a landing. A landing that incidentally would have been very close to their base at Wilhelmshaven (but not too close, because the German North Sea coast was heavily mined, which would also have been a severe tactical constraint).
There are three main answers :
- The German High Sea Fleet was in a position to threaten the Royal Navy until the Battle of Jutland (1916).
- The German had laid a very large number of mines to protect their shores (which explains why the German High Sea Fleet survived after the Battle of Jutland where they had been (almost) trapped by the Royal Navy.
- Gallipoli in 1915 was a complete disaster precisely because of the mines (as well as poor leadership), so the new front had to be opened in Greece and Serbia (with Franchet d'Esperey) rather than directly in Germany.