Germany had lost most of its available destroyers in Norway or had other ships under repair when Sea Lion was considered. Proposals to sink obsolete warships in shallow water as fixed gun batteries reveal the shortage of usable naval fire support. The invasion force would then have to move troops and supplies across a sea channel in vulnerable river craft exposed to weather and waves. The Royal Navy had spent the interwar years practising night fighting, so a daylight crossing did not solve the danger after sunset. Winning an air battle could not by itself create the surface escort and logistical system needed to sustain the landing.
Chapter · 57:49-59:01
Sea Lion's Channel crossing and German surface weakness
German destroyer losses, improvised fire-support ideas, Channel weather, and British night-fighting doctrine made the crossing far harder than a large river operation.