The horses do not swim.

They walk on the towing-path, with a rope tied to them, and the other end to the barge.

So it gets pulled along.

The bargees we knew were a good friendly sort, and used to let us go all over the barges when they were in a good temper.

They were not at all the sort of bullying, cowardly fiends in human form that the young hero at Oxford fights a crowd of, single-handed, in books.
The river does not smell nice when its bones are showing.