Among a collection of old cards, I found these - the ones that make me wonder about the stories that fit the pictures.
The first has the undated message on the back:
This ship is the one I was aboard when I first came to Venice.The Marco Polo was converted to a troop transport in Venice from 1917-1918 and was sold for scrapping in January 1922. So that gives me a fairly precise date.
The card was accompanied by several others, mainly of submarines, both Italian and British.
The submarine above I think is almost certainly a Squalo with its fairly distinctive profile. I really don't know about the second. Both PTOs say "Italian submarine".
Mounts Bay, where this picture was taken, is a bay in Cornwall near Lands End. St Michael's Mount is in the centre of the bay and I wonder if the small conical shape on the horizon could be that.
I'm sorry, by the way, for the wonkiness of the cards. Not one of them is square and it was impossible to straighten them up any more without having to crop too much.
The British L class submarines served in the 1920s, so again the date fits. The majority were scrapped in the 1930s.
Was the sender a submariner, or merely interested in submarines? And why would he be in Venice?