Haven't they always been on map and visible?
I mean it should be more like HOI3, where you can more easily sink an entire invasion fleet and the divisions that are being transported. I also think you should have to build transports and landing crafts separately from convoys.
While the interface can be a little bit frustrating to conduct an invasion, it's even more frustrating to defend against one. There's very little you can do to prevent the AI from naval invasions, and there's almost always a "naval invasion" warning on the right side of the screen.
Generally, convoys seem to be a bit too cheap in HOI4. I see AI Germany having hundreds of convoys, where in WW2, Germany's merchant fleet was virtually reduced to 0 within the opening months of the war. It's too easy for the German AI or player to conduct trade or naval invasions.
To give an example, the invasion of Norway involved something like a third of Germany's total shipping. While the invasion was a success (very much a matter of luck--it was a
very risky operation), it was a strategic disaster that resulted in the loss of 3 cruisers and 10 destroyers as well as a substantial part of Germany's transport fleet, to say nothing of the colossal resources required to garrison Norway throughout the war. From that point forward, Germany's capacity to conduct substantial surface naval operations in the North Sea disappeared.
Similarly, Sea Lion was never a serious plan. It would have taken years for Germany to build up the necessary transport capacity (victory by 1950!) to supply an army of any size across the channel. Even in the unlikely event of a Luftwaffe success in the air Battle of Britain, only a small army could have been supplied, which would have been slaughtered by local resistance-- tanks or no tanks. There simply were not enough boats in Germany.
Italy faced similar problems in East Africa and (later on) in North Africa, with catastrophic losses in shipping at a rate the Axis simply could not sustain.
Obviously the German player in HOI4 is going to want a little bit more agency than what was historically accurate (IRL, Germany was doomed to fail from day 0 of the war), but some kind of rebalance of naval invasions and convoys is warranted.
