The water temperature while I was in Izu recently ranged between 19ºC and 21ºC, mostly hovering around the lower end.
The first time I dived in Izu back in 2006 was also the first time I used a drysuit, since up until that point, I had completely avoided getting into water below 28ºC...as any sane person would do.
Having lost my vestigial grip on sanity since that time, I let my friends persuade me into diving in Izu with a wetsuit during my recent squid search.
To this end, I had a custom wetsuit made, 6.5mm farmer-john style bottom with a 6.5mm pullover top and attached hood. On the recommendation of my friends at Dan's Dive Shop, I went with the consensus top-of-the-line suit, made by a company named UGO.
(Side Note: UGO is an acronym for the company owner's name, Yuki Goto, which is kind of funny, because a few friends mentioned that UGO 6.5mm suits are considered the Ferraris of wetsuits. If you take the first two letters of the owner's name (instead of just using the letter "U"), you get "Yugo", which...for those of you old enough to remember...was the antithesis of a Ferrari.)
The neoprene used by this company is special. I don't completely understand why, but everyone agrees that it doesn't compress as much as normal neoprene, it molds to your body, and it's really warm.
Custom-tailored, the suit was a bit difficult to get into initially, but not as difficult as I had imagined. Actually, getting the suit off was more of a chore.
Anyway...I'm sold. The suit fit so well that I was almost entirely dry after I got out, and I was as toasty, if not toastier, than with my drysuit. Plus, it's a lot easier to move around in a wetsuit, as you don't have sudden shifts in buoyancy (air pockets move around inside a drysuit).
And never to be discounted is the fact that with a wetsuit, I can pee if the need arises. It's the simple pleasures that really count sometimes.
The major drawback is the difficulty of learning how to take the pullover top off. It's a tight fit, so it requires a nimble twist-and-flip technique. My initial attempts were more of a flop-and-squirm. But, on the last dive of my recent trip, I finally succeeded in dis-wetsuiting without external intervention...to a hearty round of applause from bemused onlookers.