This work is a copy from an etching by the Venetian painter Marco Ricci (1676 - 1730). Copying from works by Master artists like Ricci was a common teaching technique and it was often used to train young artists. It helped introduce them to the grand subjects and compositions of the Old Masters while developing their drawing and painting skills.
Girtin may have made this watercolour while he was a student of Dr Thomas Monro (1759-1833). The physician engaged several young artists, including JMW Turner (1775 - 1881), to make copies of watercolours in his collection between 1794 and 1796. Monro was a keen supporter of the arts and so made his collection available to young artists to help them develop.
A copy after No. XIII of the 24 Landscapes of Marco Ricci, etched by D.A. Fossati, Venice, 1763; Girtin and Loshak date No. 17 to 1798-1799 and the list five other watercolours by Girtin after Fossati.