if only you couldn't leave a strait if it's occupied by enemy. the thing is, uk can leave a submarine in the suez canal and after losing suez province that sub blocks land unit's movement indefinitely. it will run out of oil and supplies but it doesn't matter. ships do not melt away like land forces when out of supplies. that sub can float there literally for years. also it's very hard [close to impossible] to spot him in suez canal with 50% asw efficiency and outdated destroyers [the italian starting navy]. so no matter you know it's there, no matter you send all your destroyers and subs on the same sea zone with asw patrol, no matter you send naval bombers on naval interdiction mission there and no matter the enemy sub don't even have fuel to run, you won't cross the canal with land units.
last game the same problem also occurred with trapped subs in the mediterranean. uk has some subs there, gibraltar and suez is closed for a year and no allied port "inside". the trapped ships can float there for years harassing transports... even lvl2 dive boats acts like modern nuclear-based subs and can stay on open waters for very long periods.
i propose a naval unit completely out of supply should suffer the same attrition level as land units [35% per week] to overcome this issues. No military unit can operate without food and fresh water [at the very least].