This is a postcard of a painting, The Pool of London by George Vicat Cole (1888) in the Tate Gallery. The card is about 25 years later.
This view seems to be just west of London Bridge (rougly here), because if it was east, you’d be able to see a bridge. (I think the Southwark Bridge predecessor is hidden by ships.)
A pool (in this context) is a deep and still place in a river – and thus a good place to moor a boat. London’s pool is divided into upper and lower parts, which are respectively west and east of Tower Bridge. For centuries, cargo vessels traded at riverside wharves in and around the City of London. When these quays grew increasingly overcrowded many boats took to mooring in midstream and loading or unloading with the assistance of barges. The Pool was a perennial forest of bobbing masts. As early as 1586 William Camden boasted: “A man would say, that seeth the shipping there, that it is, as it were, a very wood of trees disbranched to make glades and let in light, so shaded it is with masts and sails.”
As Britain’s empire expanded and the industrial revolution took hold, the Pool became the busiest section of river in the world, crammed not just with ocean-going ships bearing exotic produce from foreign lands, but boats full of immigrants and emigrants, skiffs bringing oysters and fish from the Thames estuary or North Sea, and colliers transporting coal from Tyneside. The first police force was formed at the end of the 18th century to prevent theft and fraud in the Pool of London. Around this time the riverbanks began to fill with imposing warehouses, several of which survive, notably at Butler’s Wharf and Hay’s Wharf on the south shore of the Upper Pool. With the construction of inland docks such as West India and East India, and later Royal Victoria and Royal Albert, the largest ships found new berths but the Pool remained a hive of activity until the ineluctable decline of London as a port in the mid-20th century.