You have a choice of ferries, depending upon when and how you wish to travel. The two main ferry companies are:

ANES. fares HERE or
Dodekanissos. HERE

The ANES boat (Symi II) usually travels from Mandraki in Rhodes via Panormitis, stopping there for an hour and therefore take about 4 hours to reach Yialos harbour at 13:00. The Symi II leaves again at 16:00. giving you c. 3 hours in Symi if you are having a day trip. The Proteus has a more irregular timetable and does not visit Symi as often.

The Dodek boats are much faster, leaving from Kolonna Harbour in Rhodes and the 08:30 am ferry takes about 50 minutes. The fare is €15 each way. The Dodek Pride calls in to Yialos on the return trip at c. 17:50. Don't be late; she stays for 5 minutes only!

As always, check the ferry timetables shortly before you travel. The timetables are not the same throughout the year, to put it mildly, and may also change depending upon the weather. The Proteus is rated at Force 9 winds but the Rhodes harbour master doesn't like smaller boats travelling in lesser breezes.

Tickets may be bought online or at the ticket desk on the harbourside in both cases. Have fun. We always do.