Note: The boat shown on this coin is probably a navis lusoria. Here's a translation by Lars Rutten from a comment by a German auction house:

"The naves lusoriae were introduced around mid 3. Century AD. Those slim and flat boats were used by the Roman river flotillas until the late Roman period and were of substantial military significance. Iulianus II. Apostata, for example, managed to get his 3000 men from Ulm to Sirmium within just eleven days (during the Civil War against Constantius II.) — which means that, assuming one boat carried thirty oarsmen and twenty additional men, the emperor used 60 naves lusoriae. One of the biggest advantages of this type of boat was that it could be manned with regular land troops. On the basis of naves lusoriae that were found in Mainz, the University of Regensburg recreated a lusoria, see Ferkel/Konen/Schäfer, Navis Lusoria. Ein Römerschiff in Regensburg, St. Katharinen 2004. Postumus propagated this type of boat in an extensive way on his coins."

Here are two other coin types with this type of boat: Antoninianus of Postumus showing a personification of the Rhine; Antoninianus of Postumus showing Serapis.