Most years Fisherman's Cabin does not open until late May. One reason for this is that the access road (at Agios Georgios beach) is usually badly damaged during the winter and not repaired until then. Until it is repaired the road is just not passable by cars. The cause of the road subsidence is underground water from the heavy winter rains running down the hillside (under the ground) and causing the road to slide into the sea. They won't repair it until the ground has dried out enough for no more subsidence to occur.
Later in the season the main problems are the holes caused by scrabbling car wheels on the hills or at the edge of concrete slabs.
Personally I would say don't try to drive there in a normal hire car due to ground clearance problems. It would be very difficult not to damage the underneath, exhaust and tyres.
Leave the car at the side of the road at Agios Georgios and walk.
The last 2 years I have called in at the restaurant during mid-May while they were very busy with painting and expecting to open towards the end of the month.
Take Costas's mobile phone number with you and contact him during your stay.
See
http://www.agiosgeorgios.co.uk/fishermans.htmDennis