This is "A Dutch Man-of-war and Various Vessels in a Breeze" by Simon de Vlieger, painted around the mid-17th century. You can see a frigate battling to sail close to the direction of the wind, while others are running with it, travelling at some speed. The water appears calm despite the strong breeze, suggesting we're in relatively sheltered waters. Most likely, this is set in the Rhine delta in southern Holland, where Simon de Vlieger was born.
Notice how both ships have their sails tied tightly to one side as they try to sail against the wind. In contrast, smaller boats are having an easier time of it, using the wind to speed across the water. There's even a warship at anchor and more boats on the horizon line. This is a busy maritime scene, typical of the thriving trade-based economy in 17th-century Netherlands.
De Vlieger was one of the leading marine painters of his time, known for capturing the sea in different moods and rendering ship details with convincing accuracy. His style had a lasting impact on later artists working in this genre. If you look closely, you might be able to spot a small piece of driftwood inserted into the foreground - it's where De Vlieger would often sign his name. Unfortunately, the paint has darkened over time and it's no longer legible here.
A Dutch Man-of-war and Various Vessels in a Breeze 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.