Orientation
All hydrofoils and catamarans dock at the center of the north side of the island at the eponymous port of Hydra. Practically everything of interest to the visitor is on the waterfront, in the few main streets behind the harbor, or along the main road that follows the coast or within a block or two of it. If you head west for about a kilometer, past a little headland, you'll reach the neighboring, far less touristy village of Kamini.
Understand
Hydra is deservedly one of the most popular day-trip destinations from Athens. The port of Hydra has a scenic location in a deep harbor, with whitewashed houses rising on the hills on both sides from an azure blue sea. An enlightened policy of development has meant that there are absolutely zero high-rise houses, cars, or motor-bikes -- the only internal combustion vehicles on the island are the few city-owned trucks and a fire truck -- and while the main port boulevard is packed full of tourists in season, only a few minutes away are quiet back alleys with nary a soul, and many of those visitors are day trippers, leaving the town relatively peaceful by night.
Incidentally, the name has nothing to do with the nine-headed monster of Greek legend; instead, it's an abbreviation of Hydrea, or "spring". The spring has long since run dry, but the name remains.