The photograph shows what appears to be some sort of viewing platform, occupied by a number of people, mounted on a tower standing in water, apparently in a bay, as a shoreline with buildings is vaguely visible in the background. However, ripples in the water suggest some movement, either of the water, or of the contraption itself, and this appears to be supported by the inscriptions handwritten in purple ink. On the front, it states, "Passes between St Malo(?) & ...(?)" - unfortunately the last part of the inscription has vanished along with torn corner of the cdv.
On the reverse, also in purple handwriting, "It goes by Machinery - I passed over it last year & again this year twice Aug 1882 It takes about 3 minutes to cross its only 1 sous." I believe the handwriting is that of my gg-grandfather Henry Payne (1842-1907). This suggests that it is the contraption which moves, rather than the ripples being produced by a water current.
I knew that he travelled to the United States in February-March 1880 and had returned to England with his family by November, as described in this article. However, I wasn't aware of any trips to the continent, as suggested by the reference to St Malo. A quick look at maps for northern France suggest that the destination of the contraption could have been Dinard, situated on the other side of the bay from St Malo. I used Google Maps' very handy Street View facility to check out the geography of the bay, and it seems quite possible that contraption was travelling from St Malo to Dinard.
I wonder if any other readers have come across anything like this extraordinary contraption, and can enlighten us further. I assume it was a tourist attraction of some sort.