That is sweet that you wish to accompany your traveling companions. If their hearts are set on Berlin from Warnemunde, perhaps they should bite the bullet for the cost of the ship tour and go there without you since you've been there. As a person who coordinates private port excursions for us and our friends cruising with us, I know that I try to find a cost-effective alternative to cruise excursions when feasible, but sometimes the alternatives such as DIYing it to Berlin for a day just don't make sense.