Hi there,
I have been researching this topic and have found considerable evidence that there was a ferry at thankerton. It was the reason the bridge was built at that spot, even though it was prone to flooding. The house, Boat farm, dates back to the early 1600's and was probably shelter for the people who worked the ferry. In older books on convigton and the house of Fleming it states that the Earl of Wigton funded the ferry so the parishioners of Thankerton could journey to the Kirk of Biggar.
The daughter of James Liverance, Lilis married one of the Murray brothers, James, Adam and John who were tenants at Boat Farm.
Hope this helps x