Now days a wetsuit is only for tropical diving.
Winter/Spring in a quarry or mine - having looked at exposure times 4.5 - 10 degrees will take 5 minutes for loss of dexterity, 30 - 60 for exhaustion or unconsciousness. Even 10 - 15 degrees (most UK ocean diving) is 10 - 15 minutes and then 1 - 2 hours. I can understand getting away with a short ocean dive in the summer in a wetsuit, anything outside of that looks a worry! That is without considering ending the dive much colder than when it started and the associated DCS (those times are without thermal protection).
A club local to us still tell new divers that wetsuits are perfectly adequate...then you look at them after they have got out of the water 6 minutes later, can't understand why young people don't want to get into 'grass roots' diving!
We dive from June to October in wetsuits in the sea, then swap to drysuits for Scotland over the winter
Definitely don't doubt Dawn - not if you value your life
Diving in a wetsuit isn't particularly bad. Even in winter it's still manageable if your bottom times aren't long and you aren't hanging around in areas of freshwater run off. I swim and freedive all through the winter in wetsuits. I'll often do 90-120min in the water in Scotland in my freediving suit or an hour's swim in a 3mm steamer.
What makes it bad is what you do before and afterwards. Wind chill and evaporation will suck the energy straight out of you, especially if you can't move around. If you're shore diving and can get changed, straight in the water, out and get dry clothes on and some food/hot drinks then it's just a bit of discomfort. A cheap hivis builders jacket is brilliant for putting on straight away and a good breakfast with plenty of fat/oil. If you're sitting on a RHIB there and back, not moving, then that's a different experience all together. Personally I wouldn't use a wetsuit for boat diving in the UK except in high summer and even then if I had the choice I'd use a drysuit. If it came down to cost I'd rather have a rubbish drysuit than a good wetsuit.
Caliph Hamish Aw-Michty Ay-Ya-Bastard, Spiritual leader of Scottish State in England
Having done both, I would say that drysuit diving IS better than wetsuit diving but wetsuit diving is possible for a large part of the year.
I have dived in 8C water in a 7mm wetsuit and have been pretty comfortable with two 45 minute dives in a day (there was a large open fire on the beach for warming between dives). What does make a huge difference is how you manage the interval between dives. If you stand there in an unprotected wetsuit on a beach or boat, you will lose a lot of heat through evaporation/exposure. Warm drinks, windproof jacket, shelter etc help a huge amount.
What the biggest problem is is the cost of a new drysuit - it is not feasible for someone trying diving for the first time to go straight out and buy a new drysuit at around £700+ for a sport they might (or might not) keep doing. Managing to get a second hand suit can help a lot with this but is not always possible. This is especially a problem if, like me, you are not a "standard" or regularly available size. I am 6'3 and my back and arms are very long in comparison to my legs so a "standard" fit of drysuit normally doesn't work so there is no second hand market for suits for me.