This is "A Gondola on the Lagoon near Mestre" painted by Francesco Guardi. It's a beautiful example of Venetian landscape painting from the late 18th century. We see a serene summer scene, with gondolas gliding across the water, their oars creating gentle ripples. The passengers are sheltered under canopies, while on the horizon you can spot sails catching the breeze.
In the distance, that's the tower of Malghera, near Mestre on the mainland. You might notice a similar painting by Guardi in another collection - it's his larger "View of the Venetian Lagoon with the Tower of Malghera". To the left, you can just make out some figures, possibly fishermen, standing on a narrow strip of land.
The painting has unfortunately worn and damaged over time, but it still captures the essence of a summer day. The changing light on the water's surface is particularly striking. This work is likely to be from around 1780, which would place it among Guardi's later works.
A Gondola on the Lagoon near Mestre hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.