i think you can book them in sids,but you will have to go to kass to start the trip,
you can hire motor boats for the day from sids,but its quite windy an choppy out on the north coast,we usually drive round to ag stef north east,an hire one cos the bays are are on the leeward side of the island,with no wind an great little bays an tavernas to stop in.
erm,think it cost us about 60e to 70e for the day but the petrol cost a lot more this year,about 30 e,so all in about 100e,not cheap but great fun