The guys across the road from the Marina pool are spot on. Its all linked to San Stefanos NW Boat Hire. We do it every time we visit Arillas. For a small 10hp boat, sits up to 4, it costs about 60 euros for the day (10-6pm) plus your fuel. We have never been over 40 Euros for the fuel and always use up the full day. Last year we got water taxied to the harbour at San Stef (free) and took the Olga for the day, on our return we were told to try taking back to the beach at Arillas, but then told to take to the harbour again, this time we were given a free lift back to Arillas.
The limit of your adventure is Sidari to the North and Paleokastritsa to the south. Our personal favorite is to the south, so so so beautiful. Especially the deserted coves and beaches and the gentle lap of the water against the hull when you cut the power. The tutorial lasts about 5 minutes, dead easy and by the end of the day you feel you have owned a boat in Corfu all your life.
Have fun
Gary & Carolyn