At the top is the low profile of a coastal building, perhaps a fort. The central subject is a large, moored warship with a yacht passing its starboard side; compare the vessel below the distant castle in the painting East Cowes Castle, the Seat of J. Nash, Esq.; the Regatta Beating to Windward, exhibited at the Royal Academy in 1828 (Indianapolis Museum of Art).1
At the bottom right the occupants of a large rowing boat are shown, possibly including women; the sketch continues a little way onto folio 21 recto opposite (D18028). The slight verticals may indicate tossed oars; compare folios 9 verso and 47 verso (D18006, D18080). For more on studies in this sketchbook relating to the regatta events at Cowes from late July 1827 onwards, see the sketchbook Introduction.
Martin Butlin and Evelyn Joll, The Paintings of J.M.W. Turner, revised ed., New Haven and London 1984, p.150 no.242, pl.246 (colour).