If I need anything more than a 5mm + hooded vest, I am diving in a drysuit.
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Everyone is different in terms of thermal needs. When I am doing a 3 hour pool session with 3mm suits, almost all students need a hot tub break to get some BTUs back by the middle of the session, and I am pretty close to needing it, too. In contrast, one of our DMs regularly does the entire 3 hour session with just a rash guard. In contrast the other way, we have another DM who is shivering in a 5 mm by the time we take that break.If the plan is still to get certified in South Florida in February I would hold off on the drysuit. Aside from the cost, you're going to be extremely hot in the Keys, even in February. I just took my son for a walk and I'm sweating bullets right now. You're going to end up with a few different exposure suits anyway so I would just go ahead and pick up an inexpensive 3 mil and dive that when you're down here and you may find you can dive it for a while in the summer up where you live.
You do what you have to do between dives. It usually only takes about a minute to get it down so that it is hanging from your waist.Again, I don't have experience with a dry suit, but I'm assuming when you're down here in Florida during the summer you can easily open it up during surface intervals to cool down if you're doing repetitive dives? I also assume you're using it simply to provide a redundant lift for your doubles?
Hands down, drysuit.No saltwater in Minnesota which means that the lowest the water temperature can get is 32* -- and it does. I'm contemplating a range of local diving activities from wrecks in Lake Superior (35-40* at depth year around) on up to Lake Minnetonka where summer temps can hit 75*.
How many suits am I going to end up needing?
What alternatives are worth considering on the cold end of the scale? Is it worth trying a semi-dry suit in 7mm or 8mm or should I just save my money for a dry suit? What strategies for layering and combining suits are workable in practice?
Matters are complicated by the fact that I wear a size unusual enough that rental options are limited.
Hands down, drysuit.
For the temprature range (and land tempratur range), I think a trilam suit would be more suitable than neoprene due to the fact that neoprene get bloody hot in the summer (I do have experience with neoprene drusuit, I dive the same same conditions and a hot summer day neoprene drusuit do beceome a bit of a hassle).