The Plakias Facebook page says that if enough people want to go it can be arranged seas permitting. Looks like the only way is by getting a large group together and negotiating with the captain at the Smerna. Plakias Cruises is new this year, they are a private charter that carry up to 25 people. They say they go where the group who has chartered them wants to go including Gavdos . Those are they only two options I know of. I posted about Plakias cruises earlier this year. They have a Facebook page called Plakias Cruises.