There is a duty to to rescue at sea this is from the Red Cross web site:
The duty to rescue persons in distress at sea is a fundamental rule of international law. It has been incorporated in international treaties and forms the content of a norm of customary international law. It applies both during peacetime and during wartime, albeit with the necessary adjustments...
international-review.icrc.org
"The duty to rescue persons in distress at sea is a fundamental rule of international law. It has been incorporated in international treaties and forms the content of a norm of customary international law. It applies both during peacetime and during wartime, albeit with the necessary adjustments to take into account the different circumstances."
Now I am not familiar with the location mentioned, however, using Google Earth it is clear that Cardross is in the wide tidal part of the River Clyde, thus we are not talking a small river - more an inlet of the sea.
The rules are probably more aimed at seafarers as passenger ships have been known to wander off route to look for people in distress and no one queries their deviation from the timetables. It is accepted practice.
I would say the driver met the requirement to the letter, it might be unusual that land transport was involved and delayed as a result - but just one of those things and top marks to the driver.