Well after been looking for over 1 hour for a way to get inside and giving up, later the same night I found a way in by a total mistake!
When my friend visted my garrison/naval, he told me in skype he cannot see my ships, when I jumped on my Submarine, he fall down into the see and swimed ...
Then the penny dropped ..
So I told him to use a doable seat mount and swim at the spot, when he was at the Submarine location, I droped my self and tada!
Enjoy!
Edit: Forgot to mention, you partner must not have Submarine in his garrison ...
Here are some screenshots: