Hi D@G. You get more time on the islands when you take the organised excursions through San Stefano Travel compared to traveling on Pegasus, the local cargo boat as the boat only acts as a ferry to Mathraki, Othoni and Ericusa delivering goods and people and then returning to San Stefanos. With the organised tourist excursion the whole trip is devised to give you as much time possible on each of the islands. On the Mathraki and Othoni trip you have 1 hour in Mathraki followed by a 30 minute swim stop at Aspri Amos, a hidden bay on the island of Othoni followed by 3 hours in Othoni. On the Ericusa trip you have 4 hours on the island. You can find details and pictures of all San Stefano Travels boat trips on their website, link provided below. If you need any further information feel free to ask.
http://san-stefano.gr/trips_boat.phpKKind regards
Cerys