Kea is the Cycladic island nearest to Athens, and it belongs to the mainland's orbit: almost all its boats run to and from Lavrio, on the south-east coast of Attica. Paros, in the middle of the Cyclades, is served from Piraeus and Rafina. So going from Kea to Paros means crossing from one ferry network to another, and on most days that happens on the mainland.
Our route data lists Seajets on a seasonal Kea to Paros service, and the record for the other direction names Blue Star Ferries too, but none of it is backed by a dated timetable or a Ferryhopper page. We have therefore left out the crossing time and fares in those records. If a direct sailing appears on the booking page for your date, check how many stops it makes and what time it arrives before you rely on it.
The dependable plan has two sea legs. Kea to Lavrio runs every day of the year, with the first boats leaving Korissia early in the morning. From Lavrio you then either transfer by road to Piraeus for one of the daily ships to Paros, or, on the two or so days a week it runs, stay in Lavrio for the Blue Star ship to Syros and continue from Syros to Paros, which is a short, frequent hop.
Whichever route you choose, it is a long day. Start early from Kea, leave generous time for the mainland connection, and if you are flying out of Paros or catching an onward boat, arrive the day before.