As stated there are ferries from Hydra to Poros but there are none from Poros to Nafplio.
You'll either have to take a bus which involves switching to another bus then on to Nafplio which could take more time than you think waiting for the bus to Nafplio.
The other option is by a private transfer which will probably be very pricey.
If you are on Hydra there is a travel agency in Nafplio, Pegasus Travel that does day trips from Tolo, near Nafplio to Hydra and back. You could check with them to see if you can book their ferry from Hydra to Tolo which will then be a short bus ride to Nafplio.