Ferries: From Aktí Miaoúli quay in Piraeus, gate E8, hydrofoils or catamarans sail for Hydra (Ýdra in Greek) almost hourly in peak season, but typically just three times daily October-May. Hellenic Seaways (hellenicseaways.gr) provides service all year; online bookings are best, pick up tickets at its booth on Aktí Miaoúli. During high season Hellas Speed Cat (speedcat.gr) also sails twice daily (once only Tue/Wed) for about the same price.
If – and only if – you're on a driving tour of the Peloponnese, there is a better alternative: Hydra Lines' small, very inexpensive boats Freedom II and Freedom III, which ply up to 12 times daily in season (less on Sun) from Metóhi Thermisías, a tiny port (with no bus service) immediately opposite Hydra on the Peloponnese, 25km east of Kranídi or 25km west of Galatás. Leave your car in the parking lot with the Hydriots' vehicles; see current schedules at hydralines.gr. The Freedom boats have competition now from the Metoxi Express (hydracelebrity.gr), travelling 6–7 times daily in each direction from early May to early September, dropping to 3 daily during off-season. Prices on the two lines are comparable; 15min journey time.
Once on Hydra, it’s shanks pony or taxi-boats – no buses, no scooters for hire. Baggage is transferred to the better hotels by hand-cart, while taxi-boats (€3 one way) ply as far as Plákes Vlyhoú with its hotel from early in the season. Only from late May to early September are little scheduled boats to Bísti beach (€10 one way) at all reliable, leaving at 11am–noon and returning around 4pm. If you choose to charter one of these specially, it's a very expensive undertaking unless you get a group together.