All the references I have got place the instrumental firmly in first place when describing mode of transport/locomotion. I have always used it when visiting Russia and have never been corrected. So: пешком, верхом, морем, сухим путем и тоже автобусом, самолетом, поездом итд. I feel the use of the prepositional, preceded by на/в, would carry a very much more specific meaning where location in or on a mode of transport is important. Anyone care to comment?