The Russian player needs a destroyer to lift you from the convoy zone because you have a sub there. Submerged sub in a convoy zone works.
“Submerging” is a term that can occur only in a battle as an option for Submarines to “retreat” while remaining in place. After the battle is over, they are no longer considered “submerged”. “Submerging” has no bearing at all in any other turn phase.
In addition, if you have any warship in a convoy zone during the collect income phase of the enemy’s turn (that controls territories adjacent to the convoy zone) they deduct their income REGARDLESS of whether they, or ANY another ally, have ships in that zone. Japan could, for example, mobilize as many ships as they wanted in seazone 6 but it would NOT prevent convoy interdiction if an Allied power had ships in that space.