xyrandomyx
Contributor
I'm just an OW diver but I believe that if water temperatures are below 20C both instructor and students should be wearing drysuits. (7mm would be a compromise).
My rationale is very simple: diving should be fun for both students and instructor.
Being cold is not fun - the added stress induced by cold water is a safety concern and I don't think a student can learn that well when all they want to do is get in and out as quickly as possible and get the dives over with.
I also think that instructors in cold water locations should be actively encouraging their students to dive in local waters - it can be fun for the student and good for an instructor's income if they are not losing their students to instructors in warmer climates. If the experience for the student is a miserable one due to climate then surely investment in some drysuits is not a bad idea for larger shops.
I appreciate it might not be practical for some independent instructors to own drysuits because of the cost and having to cater for a multitude of fits but I don't think training in anything less than 7mil in cold water is a good idea either so either get 7mm/drysuits or reschedule the trainng.
I've dived in 20C with a one-piece 5mm wetsuit and a hood. I was very much comfortable for two 50--60 minute dives with an hour and a bit surface interval in between. Which illustrates the point others have made: the instructor's not going to feel what their students feel anyway, because everyone's resistance to cold is a bit different. From a student's perspective, I can't see why an instructor shouldn't wear a drysuit if they're either particularly prone to getting cold or they're in the water longer than their students.