Wlathers used to have a railroad barge, tug boat and barge loating apron . I haven't seen these currently, but Walther's tends to run these things in batches.
There is a book on scratchbuilding with styrene that documents building an SN3 car ferry.
I have seen an article that documented a railroad where a barge dock was used for interchange. The author had modeled several barges, and had them mounted on rolling tables, just high enough to Dock at the apron. He could start a operating session with a loaded barge at the apron, pull those cars off for delivery, load others on the barge, and roll it off, and replace it with another one.
This would be a very useful method for doing interchange on a small layout, as it would take up a lot less space than an adequate staging yard would .
It could be done with some very small railroads. There was a small iron ore RR in Tennessee that was built in two sections with barge traffic in between, with the mines on one end, with one locomotive, and the furnaces and the interchange on the other, also with one locomotive.
I have a riverboat on my current RR, space is really tight on that end of the layout, so there is no room, but a rail barge would have been a very cool addition.
Bill Nelson