I'm planning a trip for the summer of 2018. My friend and I are two young American girls. We would be staying on Hydra for a few days, and then want to go to Zakynthos. I know Hydra is accessible only by ferry, and that there are buses from Athens to Zakynthos, but we'd prefer not to backtrack all the way to Athens. Are there any buses or other means of travel (safe for young foreign women) that would be more direct, like one out of porto heli?

Slight edit, I know there are buses from Athens to Kyllini, where we can then get a ferry ride to Zakynthos

The best and fastest way is to get back to Athens and take a KTEL bus to Zakynthos, everything else would be too complicated and too time consuming.
Here is the website of KTEL Zakynthos :
http://ktel-zakynthos.gr/en/routes/s...hina-zakynthos

Easiest way: via Piraeus - Athens Kifissos bus station or airport.
Buses from Atehns to Kyllini ferry leave at 6.00, 7.15, 7.30, 9.00,12.15, 12.30, 17.00, 17.30.
If you don't want to travel through Athens, you may go by fliying dolphin up to Poros, by local ferry to Galatas and from there by bus to Isthmia whre you can change to one of the Athens - Kyllini buses which stop there.
According to the actual timetable, buses from Galata to Isthmia leave at 11.10 and 17.30. In order to catch them, you have to leave Hydra by hydrofoil at 7.05 or 15.10. With the first bus, you can probably catch the zakynthos bound bus leaving Athens at 12.30 and arriving at Zakynthos around 17.30. If you take the second bus, you will most probably have to sleep at Patras.
As an alternative, you may also travel via Ermioni - Kranidi - Isthmia, but the connections are not better.