Dumbrock Loch in Milngavie.
You can park at the car park in Mugdock Country Park which is just opposite the loch. Cross the road and you can then head through a gate and follow a footpath north to the small loch. It's a lovely place which is perfect for wild swimming and BBQ.
The loch isn't visible from the road. To access it, you'll need to go through the gate into the field across the road from the entrance to the visitor centre. You'll then see a path - follow this through the field until you come to a second gate. Go through the gate, then over a small stream and follow the path down to the loch on the right hand side.