Irish should know that Philippines ferry websites are totally unreliable. Even Ocean Jet's which is more reliable than any of the others.
there is two ferries, the fast Ocean Jet one via Tagbilaran and the slow direct coast-hugging one, which I think is run by Cokaliong (though not sure). I took it once about 10 years ago and I know that at least up to about two years ago it was still going as I was in a ferry booking shop then buying a ticket for another ferry and saw it advertised. It takes about five hours. It has the advantage of dropping you directly at the Dumaguete city port. I would imagine the fare to be in the 300-400 peso range. As always to be sure about schedueles and prices, just buy a copy of the Cebu Daily News, look at the ferry advertisement page, and ring them up. Or if you are in Colon, go into one of the numerous ferry booking shops on Colon St, and ask.
really the best way to do this surface is the conventional bus and ferry route.