Convened by the Eastern Orthodox "Pope", the Ecumenical Patriarch Bartholomew 1st, and Prince Philip, the Duke of Edinburgh, the pilgrimage has marked a breakthrough in relations between religion and ecology. For the first time leaders of the Christian church have declared pollution and the destruction of the environment to be a sin, at a symposium aboard the ferry as it sails towards the island where the imprisoned St John the Divine had the vision that forms the final book of the Bible.
The pilgrimage, calling at Athens, Istanbul and Ephesus as well as Patmos has the backing of the Archbishop of Canterbury,George Carey, Nobel Peace Prize winner Archbishop Desmond Tutu, US Vice President Al Gore, Jacques Santer, President of the European Commission, and Boutros Boutros- Ghali, the Secretary General of the United Nations.
Some 200 leading scientists, environmentalists, politicians and clerics,including much of the hierarchy of the Greek Orthodox Church, are on board and participating in the symposium.
Patriarchs in black stovepipe hats and long white beards mingle with the likes of David Bellamy, the almost equally hirsute environmentalist and television presenter, and Tim Worth, former chairman of the Clinton- Gore presidential campaign and now Under-Secretary of State for international environmental issues.
A detailed exposition of the latest science on global warming by Sir John Houghton, the chairman of the Royal Commission on environmental pollution and adviser to John Major, is closely followed by a female chief from Western Samoa - who is also head of the rugby union board - sprinkling water on the Eastern Orthodox Pope in a traditional Pacific blessing.
The newly elected Bishop of London, Richard Chartres, rubs shoulders with composer John Tavener who is staging a performance of his oratorio on the Apocalypse in the ancient theatre of Ephesus tonight.
Roman Catholic Cardinal Roger Etchegarry, the head of two pontifical councils, dines with a member of the Ibalot-Igorot, indigenous people of the Philippines, and the leading industrialists of Muslim Turkey
The Ecumenical Patriarch - an approachable, bespectacled somewhat tubby figure who refers to the ferry as "a latter day Ark " - warns that damage to the environment may be about to precipitate an apocalypse
Ecological catastrophe is "a great threat," he says, "a great danger not only for the faithful of our Church but for the whole of human kind".
Over a dinner of lobster and locally caught fish, the Patriarch pointed in the air with a self-deprecating smile when I asked him how he had become interested in the environment
"By inspiration," he said. He is determined to put the issue towards the top of his church's agenda, describing it as a matter of "life and death". He brackets pollution with "nuclear destruction" as a possible cause of the end of the world.
Both he and his leading theologian, Metropolitan John of Pergamum, say that Christianity's focus on humans and their mastery over nature has greatly contributed to the crisis. The Metropolitan, who also serves as "foreign minister" to the Patriarch and other leading Greek Orthodox clerics, described pollution and environmental destruction as a "sin"
"We are used to regarding sin mainly in social terms" the Metropolitan told the symposium as the ferry sailed past the barren eroded hills of the Dardenelles.
"But there is also a sin against nature. Evil is not only a matter for human beings but affects the entire Creation. Any pain or suffering inflicted on non-human beings is no less evil than it is when applied to humans.
"This morality still awaits to find its place in our Christian consciences."
He added "The Book of Revelation is full of references to the consequences of evil for the natural environment .The ecological crisis, more than any other problem of humanity,reveals the truth that the world forms one unity and one community and that even the slightest violation of nature in one paret of the world leads inevitably to consequence affecting in the rest of the world."
Prince Philip,who has not himself joined the pilgrimage,sent a message warning that "humanity is facing an apocalypse of its own making", while Bishop Chartres spoke of the "havoc wrought by military and economic elites that devastate the earth and sea".
The theology of the Greek Orthodox Church is particularly environment- friendly, more so than other churches. It emphasises the sacredness of all living things, as well as the importance of looking after bodies which it believes go to heaven as well as souls. But it was the last Patriarch, Dimitrios who died in 1991, that began to "green" the church.
Both he and the present Patriarch, were particularly upset by pollution of the Black Sea, now one of the most endangered in the world. Over the past decade pollution entering the Black Sea from the Danube alone has increased tenfold. The sea's once abundant fisheries have been decimated while cholera epidemics sweep around its shores.
The pilgrimage was also envisaged as a way of building bridges between historically hostile Greece and Turkey - but here it is likely to prove less successful. Fundamentalist Muslims in Turkey have furiously attacked the enterprise as a Greek plot and though prominent Turks joined the pilgrimage in Istanbul the Greek government increased the temperature by having one of its warships give the Ro-Ro ferry a 21 gun salute as it entered disputed waters in the Aegean.
The Greek Prime Minister, Andreas Papandreou, and some of his Cabinet colleagues will join the celebration of the anniversary in Patmos. Meanwhile his controversial wife Dimitra - "Mimi" - threatens to add her own twist to an already tortuous situation by setting off to the island for the anniversary in her own boat, with her own entourage, to bolster her career.