Final answer:
The term 'sea defences' can refer to various efforts, therefore exact dates cannot be provided without more context. Notable efforts include the Allied minefields in the North Sea in 1918 and the invasion of Hitler's Atlantic Wall in 1944.
Step-by-step explanation:
When discussing sea defences, it is essential to specify the era and location being referred to, as sea defences have been created and evolved over many centuries in various regions. Within the context of World War II and specifically the battle against the German forces, the term 'sea defences' could be related to different initiatives.
For instance, during the first few months of 1918, sea defences in the form of minefields were established by the Allies in the North Sea as a strategy to contain German U-boats and protect the Atlantic. Elsewhere, on D-Day, June 6, 1944, the Allies managed to overcome Hitler's 'Atlantic Wall', a massive coastal defence stretching over 2,400 miles, in what was the largest amphibious assault in history. This implies that the sea defences, at least in the form of Hitler's Atlantic Wall, were rendered ineffective by the end of July 1944.
Furthermore, as an example unrelated to warfare, conservation work on Knossos, which might be considered a form of protection or 'defence' for historical artifacts against the elements, took place from 1922 to 1952.
Therefore, to give a long answer to when work on sea defences started and was completed, one would need to clarify the specific historical context, whether it is in reference to wartime efforts or cultural heritage conservation. Without specific details, the question remains too broad to provide exact dates.