Allow sea units to enter and exit land territories with a friendly port.
‘docked’ sea units on land participate in land battles as if they were normal units. They roll dice, and can be treated as casualties like any other unit. Docked sea units require an infantry you control to be present.
This allows Germany to hide his navy, and threaten Britain from the safety of the Kiel harbor.
You could also create a canal that requires the player to control either Kiel OR Denmark/Norway to move through SZ 10 to SZ 11.