Christos Akliros operates a daily shuttle service between Hydra and Ermioni.
The boat moors on Hydra just in front of the Roloi Cafe under the clock tower in the main harbor. In Ermioni, the boat moors at the main pier, where the hydrofoils debark.
Tickets (€7.50) can be purchased on the boat, or in Ermioni, they can be purchased at the Fun In the Sun travel agency (this is also the Hellenic Seaways and the Pop’s Cars agent).
For up-to-date departure times, see the Hydra Tours Travel website.