Quero has as usual covered the issue pretty thoroughly. I'll just add one point - knowing what to expect underwater can be very helpful on a fast drift. I remember drifting at 5 knots off the west of Scotland, and although I had a certain amount of manoueverability the visibility wasn't that good, and by the time I saw anything it was too late to avoid it. So tucking my hands and feet away and ensuring I remained vertical (to protect my head) were prime considerations. Get a thorough briefing from an informed local.
I've never been caught in a down current, but I've seen it happen. Two divers who had chosen to ignore a thorough and very clear briefing went too deep at Ras Mohammed in Egypt and were caught in a severe downcurrent. When their rescuer caught up with them they were at 58 metres and still descending fast, totally oblivious of their situation. The rescuer was a highly experienced instructor friend of mine who was diving Nitrox 32% and put her life in great jeopardy. Had it been me I would have left them to their fate - it would all have been over within 5 minutes.
I've never been caught in a down current, but I've seen it happen. Two divers who had chosen to ignore a thorough and very clear briefing went too deep at Ras Mohammed in Egypt and were caught in a severe downcurrent. When their rescuer caught up with them they were at 58 metres and still descending fast, totally oblivious of their situation. The rescuer was a highly experienced instructor friend of mine who was diving Nitrox 32% and put her life in great jeopardy. Had it been me I would have left them to their fate - it would all have been over within 5 minutes.