There is a Google map for this event.
Monkton Combe School boathouse is not easy to find, so you are advised to print out a large-scale version of the map.
If coming by car, get as far as the viaduct at Monkton, and turn off towards Bradford on Avon at the traffic lights. Go down the slope, and after about 100 yards, turn left. You can park at the Marina car park, or if it is full drive along the old railway line which goes underneath the A36 and Park above the school playing field.
Then walk through the marina car park and keep going for about 400 m until you reach the aqueduct. Turn right here, go over the aqueduct and then take the steps down to the river. The boathouse is at the downstream side of the aqueduct.