Ios can be reached best from Santorini , Naxos , Heraklion - Crete island or directly from Piraeus.
Ferry boats and high speed catamaran services run daily from Piraeus.
All domestic ferry schedules is here
If you plan on travelling to several different islands, it is wise to book all your ferries at once for a possible discount. however, always keep in mind that pickup and arrival times are subject to change at anytime