Just wanted to give a valuable tip to passengers travelling to Sifnos, Serifos, Milos: Do not travel with the Superjet high speed ferry! There are better, more comfortable, safer options! They have strong northern winds in the aegean in July and August and this ferry is tiny and shakes A LOT! Almost everybody got sick (adults and kids) and puked. It is forbidden to go outside for air and I can't describe the nausea and disgust we felt inside that little ferry in strong wind! Our seats were at the back and smelled fuel all the way -including other smells- and reached our destination an hour late, 4 hours that is!
People on TA warned me on one of my posts about the Jets but I already booked our tickets and couldn't cancel them. I wish I knew beforehand it would save me a lot of hustle and dizziness and headache and stomachache! Notice that I went prepared with sickness pills and all! And I never get seasick!
I recommend the Speedrunner IV also high speed but bigger. Luckily, by chance I booked that one for our return journey (because of convenient time again) and it was fantastic with the same weather (6 beaufort). There's also another option, Andreas Kalvos or Adamantios Korais, not high speed, they take longer but save you money and you get to your destination in good health!