It is not a free drop - by definition a drainage ditch is a water hazard and apparently it was marked as such. So a drop would definitely include a penalty stroke.
The applicable rule for hitting the ball out of the hazard is below:
14-6. Ball Moving in Water
When a ball is moving in water in a water hazard, the player may, without penalty, make a stroke, but he must not delay making his stroke in order to allow the wind or current to improve the position of the ball. A ball moving in water in a water hazard may be lifted if the player elects to invoke Rule 26.
Penalty for Breach of Rule 14-5 or 14-6:
Match play - Loss of hole; Stroke play - Two strokes.
Not really sure whether "waiting downstream" would be the same as "delay making his stroke... to improve the position of the ball". Probably not, but a judgement call.