The Greek ferry companies providing e-tickets are: Blue Star Ferries, Minoan Lines (domestic routes), Seajets, Golden Star Ferries, Alpha Lines, Anes Ferries, Dodekanissos Seaways, Triton Ferries, Saronic Ferries, Sea Speed Ferries, Lefkimmi Lines, Skyros Maritime Company, Ionian Seaways, Kerkyra Lines, Sarris Lines, Makedonia IV, Goutos Lines, Sebeco Lines and Kerkyra Seaways.
Many Italian and Spanish ferry operators also offer e-tickets (e.g. Trasmediterránea, Balearia, BluNavy, Corsica Ferries, Grandi Navi Velocci, Trasmed GLE, Travelmar, Medmar, Trasmapi, Lineas Romero, Mediterranea Pitiusa, Ichusa Lines, Liberty Lines and others).
Printing e-tickets is optional, which means that it is not required by most ferry companies except for Fast Ferries in Greece. Therefore, if you are traveling with Fast Ferries, you have to print out your e-ticket. If you are not traveling with the aforementioned ferry company, you can just store and present your e-ticket on your mobile device or tablet when boarding.
To check in online and get your boarding pass, you can click on the links provided in your booking confirmation email. You can either download your boarding pass on your mobile device or print it out and bring it with you.
The most important piece of advice is to make sure your device is fully charged; we all know that having your phone run out of battery is not very pleasant - especially while traveling.