I just traveled this morning via Sea Jet ferry from Santorini to Mykonos. The clerk at the ferry office told me no testing was necessary for this because it was island to island... If I was traveling TO ATHENS (mainland), then a test would be necessary. I didn't need to fill out any paperwork other than purchasing the ticket online. I purchased the 6 covid tests online and brought 4 with me on this trip just in case they changed the rules again. Hope this helps.