From Kos

From the port of Kos town to get to Nisyros one can arrange to take the Blue Star ferry that runs every Monday morning around 01.00 or Friday morning around 02.00. From the port of Nisyros the departure is on Monday evening at around 20.00 and on Friday at 23.30.

While the Panagia Evaggelistria ferry departs from the port of Kos near the PPC DEH on Monday, Tuesday, Thursday and Friday at 2.30 p.m., arriving in Nisyros at 5.30 p.m. after about a two-hour journey. The travel agency where to buy the tickets is ADRIS near the town hall . From Nisyros the departure follows the same pattern with departure at 07.00 and arrival at the port of Kos at 09.45.
In addition you can opt for the fast catamaran Dodekanisos Seaways Pride from the commercial harbour on Tuesdays and Wednesdays at 16.00 with arrival at the port of Mandraki Nisyros at 16.50 after about 50 minutes. While from Nisyros the catamaran departs at 10.45am also on Tuesdays and Wednesdays. Tickets can be bought at harbour’s tickets office.

In summer time, near the airport, the touristic boats sail everyday from the harbour of Kardamena mostly from 09.00 and 11.00, with return from Nisyros respectively on 15.30 and 18.00 , the travel time is about 45 minutes.

From Rhodes

The main ferries are the Blue Star ferries on Monday afternoons at 16.00 and Friday afternoons at 19.00, the journey time is about 4 hours, and especially in the summer you can enjoy the ferry ride through Symi and Tilos skimming the coasts of Turkey. Arriving at the port of Mandraki at 20.00 and 23.30 respectively and then continue towards Kos with final destination Athens passing through some intermediate islands

In addition, the fast catamaran Dodekanisos Seaways “Pride” departs from Rhodes port “Kolonas” at 08.30 on Tuesday and Wednesday and arrives at Mandraki Nisyros port at 10.45 and then continue towards Kos-Kalymnos-Astypaleia.

Other travel possibillity exists but are too much imprevedible to be planned correctly.