I helped a deliver a boat from the Ionian to Athens a couple of weeks ago. We had to go round the Peloponnese, which felt particularly silly given that it was the second time I had to make this trip the long way round after buying my own boat in the Ionian three years ago. It frustrates me that I still haven't been through Corinth in a boat. For what it's worth I doubt it will be open on the 1st of May, but it wouldn't hurt to give them a ring a day or two before just in case.