Downstream dives with current are hectic.
I did a dive on Saturday at Casa Cenote between Tulum and Akumal. There is a tunnel under the road from the Cenote to the sea, about 400' in all.
The current was so strong I had to hold onto a rock to avoid passing my buddy laying the line. We shot through this cave in about seven minutes. The return trip took nearly three times that long, a total dive time of 27 minutes. Overall rating of the dive was "scary as F**K", but in a good way. Knowing that the worst thing that could happen is that we may have to walk back to the cenote via the resturaunt, made it OK. It was kinda like riding a roller coaster while underwater, while underground.
I really dont envy all you florida guys the high flow systems I hear you talking about, but this is a great example of when you should change the rule of thirds to the rule of eights or twelves. Something I think should be highly emphasized in the intro course, and that Intro divers should be highly cautious of.
Downstream is more dangerous, and therefore should be considered in many cases, not appropriate for intro divers.