The schedule is seasonal, so check their website for up-to-date times of departure. The seamen’s strikes that often stop the flying dolphin and cat services generally do not affect the services of the water buses and sea taxis. If severe weather is interrupting service, however, you should probably call directly to ensure that the boats are running. All up-to-date contact info is listed on the website at HydraLines.gr
If the Freedom bus service can’t accommodate your itinerary, you can hire a private water taxi to cross from Metoxi. The general number for the Hydra taxi rank is 22980-53690. If you are arriving outside normal working hours, you’d be well advised to make arrangements well in advance.
Again, if inclement weather is threatening to interrupt boat service, you should call to make sure that taxis are running. If the Port Police close the harbor, all boats will have to stop running regardless, and some may opt not to of their own accord.