The photograph was taken from the Clyde Walkway, a long-distance footpath. This section is prone to flooding (compare
NS7654 : Flood debris by the River Clyde); there is an alternative seasonal route which bypasses it.
However, the pool visible in the foreground is permanent, and is shown on the map.
The structure in the distance is the Belhaven and Stenton Mausoleum. It stands in an old cemetery which was associated with the ancient parish church of Cambusnethan, founded in the eighth century. The cemetery fell into disuse in the seventeenth century; see
Link (at Historic Environment Scotland) for details.
The mausoleum is dedicated to Lord Belhaven and Stenton (1793-1868); see
Link (at Historic Environment Scotland) for architectural and other details. Specifically, the mausoleum was created for Robert Montgomery Hamilton, 8th Lord Belhaven and Stenton (the "Mortimer" at the last-cited link seems to be a typo for "Montgomery").
He was one of the representative peers of Scotland from 1819 to 1832 (all English peers had the right to sit in the House of Lords, but only a small proportion, selected by election, of the Scottish and Irish ones).