Think of Turner, and you think of the sea. Beaches, ports, sunrises, sunsets, raging storms, crashing waves and heavy battles, conjured up in a swirling mist of colour and light. You might be surprised to learn, then, that the National Maritime Museum’s current exhibition is in fact the first major one dedicated to this theme. The sea has been a subject of fascination for many of the greatest artists, from Brueghel and Rembrandt to Signac and Monet. But it was Turner whom the subject gripped with the most fervour, pulling him in with its charms and relentless metamorphosis. Watch the sea from a cliff top for an hour or two…