Hellenic Seaways are one of the main carriers between the Greek islands and the mainland. They have a huge number of routes. Hydra is on the route that also includes Poros, Spetses, Porto Heli and Ermioni. What is included on the route depends on the time of day that the flying dolphin or cat leaves however Hydra is such a popular destination that it is included on most trips anyway.

Flying dolphin and flying cat hydrofoils