The small ferry that connects Ocho on Osakishimojima Island with Akashi on Osakikamijima Island is only a ten minute journey and the distance is probably less than 3 kilometers.
It passes right under the Okamura Bridge, the last of the seven bridges that connect the islands along the Tobishima cycle path.
I recently published a piece on that cycle route over at JapanVisitor..... https://www.japanvisitor.com/japan-city-guides/tobishimakaido
Like all the short ferry journeys throughout the Inland Sea area, the views are pleasant and ever changing......