We all know why the Seamammal could not work in WW2-the RAF, the fact the German Navy was far too small, ect. But how about one early on in WW1, either the Germans attacking the UK or the UK attacking Germany.? At the start of WW1 air combat was in it's infancy, and both sides had large navies.
King Thomas
Some people had ideas of an operation in the Baltic which fortunately never took off. Even apart from the problems of getting through the Danish Straits, you would need to maintain a powerful fleet in the Baltic, fairly fair from its bases and exposed to continued attack by subs, torpedo boats etc.
Similarly I have seen suggestions of a German invasion of Britain but think they rarely end well for the Germans. At this point, for 1st world European nations with dense populations and transport networks, forces can be moved within the country quickly and easily. As such, even if a successful landing is made the defender can reinforce the front a lot easier, especially given the problems of supply a bridgehead by sea. Furthermore, even if some disaster meant the HSF gained numerical superiority over the GF the former would still have problems maintaining such an invasion. They need to have forces present to prevent naval bombardments, along with protecting convoys bringing supplies and reinforcements. The old problem of them needing to be strong all the time while the defender can attack at a time/place of their choosing. Less important when a bridgehead is smallish but if it was to grow.
As such, if the Germans were mad enough to try, unless they got very lucky, they would be likely to lose both all the invasion force and a lot of the fleet and merchant ships transporting, supplying and defending it.
Steve