Why during the first world war, did the entente not try to do an amphibious invasion of northern Germany, surely it was a feasible idea and could have been carried out?
Here's an answer I've given for a similar question. As to your question itself, while a landing on the Baltic or North Sea Coast of Germany was considered, it was neither considered 'feasible' nor could it have been carried out. Gallipoli demonstrated just how fraught with risk amphibious operations were, and that was simply landing in the Dardanelles. Landing on the urbanized northern coast of Germany, facing the High Seas Fleet, U-Boats, coastal guns and minefields would have been a very different story.
The closest the Entente (Allied/&Associated) Powers came to an amphibious operation after the Gallipoli fiasco was Operation Hush, which was to follow up the BEF's Ypres Offensive in 1917; the operation was scrubbed when a break out from the Ypres Salient was not forth coming, and the weather worsened.