You could travel through the Peloponnese via Tripolis but the is time consuming (but scenic). There may also be a possibility of picking the Athens-Zakynthos bus up at Corinth.
Thanks for the information!
Could I go directly from Nafplio to Zakynthos? Where could I check the bus or ferry schedule?
You can not go directly, not by bus, not by ferry. Check AjarnAndy's answer, he's got it right!
Bus schedule from Nafplio: www.ktel-argolidas.gr/…
You change bus in Isthmos on the way to Athens.
The bus will be in Isthmos about an hour after it leaves Athens.
You MUST check the routes/times with KTEL (the bus company) when you are in Greece, as the internet sites are not always updated immediately if a route changes.