Something to keep in mind if you think we might be looking at the surface of a body of water, these are the winds speed that day as per https://weatherspark.com/h/d/38062/...-1990-in-Pitlochry-United-Kingdom#metar-20-50
13 mph is between 3 and 4 on the Beaufort scale (gentle to moderate breeze). Not exactly a millpond.
If it's a hoax, there's no reason to suppose it was taken on the day, or at the place, claimed. I think taking the witnesses at their word is fine, as a starting point, but their credibility is obviously destroyed if they intended a hoax.
Clarke interviewed staff at the hotel where the men worked, and knocked on doors in the area, and nobody had any recollection of anyone talking about UFOs at the time.