You can use the website https://www.ferryhopper.com/en/ to search for ferries timetables and prices on the Volos→Skiathos route.
But always book ferry tickets directly with the ferry company and not with a 3rd party reseller.
This allows you to have direct communication with the company in the event of a schedule change or ferry cancellation.
In May, the ferry companies operating the Volos→Skiathos route are:
- Seajets with the “Superstar” ferry: 1 daily ferry around 7:30 a.m.
https://www.seajets.com/
- Hellenic Seaways with the “Express Skiathos”: 1 daily ferry around 8 a.m.
https://www.hellenicseaways.gr/en-gb/home
The trip takes approximately 2 hours
The Aegean Flying Dolphins ferries that operate this route only carry passengers (no cars)
From February/March, regularly visit the company websites to check for updated schedules
On their respective websites Seajets and Hellenic Seaways allow you to book your tickets online. You will receive the E-tickets by email in the form of a QR or bar code to present upon boarding.
- During the booking process you will be asked for the car's license plate number. If you don't know it, enter "RENT123"
There is also a selection for the car type depending on the car length. A rental car is normally in the first category (<4m25).
With a 2h crossing, for passengers take deck tickets (economy) you can sit wherever you want, in the lounges or outside decks
You will not have access to the car deck during the trip. So take with you what you will need during the crossing.
- Be at the port approximately 1 hour before the ferry departure.
The ferries you will take are maybe bigger than those you know in the USA, Around 1500 passengers and 200 cars/trucks
When boarding the car you will be guided by the ferry staff.
See this fairly representative video of the boarding process:
https://www.youtube.com/watch?v=0BU7fOu_IfM
When renting a car, always tell the rental company that you will be taking the car on a ferry. Rental companies may charge extra for insurance reasons.