"Mercer's Pool" was a large, natural pool in the river Burn that flowed past ( and still flows past) Brentor, and it was a popular venue in the period of my youth (late 1940s/ early 1950s) during the long hot summers that only occurred during those halcyon days of memory.
I have no idea why it was so called. Is there any record of a person / family of that name with any connection with Brentor during the period, say, between 1930-1950? It is not a name that is known to me in any other local context.