I have wet, semi-dry, and dry suits. I switch between each depending on weather and water temperature.
Wetsuit:
When I purchased my wetsuit, I also purchased a matching shorty designed to be worn over it for extra thermal protection. The shorty came in 2 versions, with and without hood. I purchased the model without hood so that I could also use the shorty for very warm water diving, and I generally prefer a separate hood as I find an attached hood to be a bit restricting.
What I found when diving my wetsuit + shorty is that when diving temperatures where the combination is needed, I am never truly warm, and I fatigue much quicker, especially out of the water and doing anything on the surface as the layers of neoprene are bulky and restrictive. I found it was not worth the hassle to were this 2 layer combo due to the bulk and impact on range of motion while still not being totally comfortable (warm) during the length of a dive (30 to 60 minutes).
I have even added a formfitting 2mm neoprene pullover vest type top to wear on my upper half as a base layer under my wetsuit. but that only adds to the bulk of the layers when combining the other 2 pieces, and only improves things marginally.
Semi-dry:
A small handful of years ago, I was offered a great deal on a nearly new semi-dry duit. As I have posted in other discussion threads a few times, my suit fits and seals so well that I could probably add a chest inflator and upper arm dump and dive it as a drysuit. I like diving my semi-dry when the water is cold but the air temp is warm, and my suit does a great job of keeping my comfortable throughout the length of my dives....plus, since my suit keeps me relatively dry, I am also comfortable when I get out of the water heading back to the car, instead of shivering uncomfortably as I would when peeling my wetsuit off.
A semi-dry should fit snugger than a wetsuit per-se...it should hug your body comfortably snug without causing any major restriction. The body hugging nature will be more restrictive when donning the suit but that sensation should ease away quite a bit when in the water as the suit material tends to relax a bit when wet.
The biggest concern once should have when shopping for a semi-dry is fit. It really does need to be comfortably snug all around. Any pockets of air or gaps arounds the body will reduce its ability to keep one comfortable. The second concern is how well the seals at the neck, wrist, and ankles fit, as they play a major role in limiting the ingress of water into the suit. I had the neck seal of my suit altered as it was initially too big which allowed cold water to flow down my neck and back. If you find a suit that fits well but the seals are too large, then inquire if they can be altered, or find someone to do it after you purchase it...this can also be a fairly straight forward DIY project if you have that penchant.
Drysuit:
I spent the majority of the last 10 years living in Belgium. Unless one has quite the constitution, the wetsuit season is quite short, at least for me. The diving in Belgium is mostly quarries and a few lakes, and the water temp only gets to about 68F for the better part of 3 weeks in late July/early August. After a few years of diving in my wetsuit/shorty combo, I got tired of feeling like the Michelin man while still being cold, and my season was cut short as it was just too cold for me to dive comfortably prior to the beginning of June or past mid-October. This was a dilemma because my preference was to dive often, and there were less folks crowding the dive sites when the water was colder, so visibility was better as there were less people stirring up the bottom.
The solution to my dilemmas of a truncated season, and quest to be comfortable in the ever present cold water was found in purchasing a drysuit. While a drysuit is not the end-all be-all solution for every dive scenario, it provides relatively unparalleled comfort (warmth) when diving in cold water. It provides the ability to comfortably do multiple dives in a single day....there are few things worse, in my opinion, than pulling on a damp wetsuit for a second dive on a cold day, regardless of water temp.
The major drawbacks of a drysuit are 2 fold:
1. The cost of the suit - drysuits tend to be more expensive than the other options available, and there may be ancillary expenses such as the possibility of needing new fins with a larger foot pocket to accommodate boots.
2. Because it is the air in the suit that keeps one comfortably warm, one has to re-learn their special awareness/how to control their body position in the water. It kind of feels like learning how to dive again as initially there tends to be a lot of flailing around like a novice diver....but this is surmounted rather quickly if one is able to get in the water (pool/confined water preferably) often to work on this, my guestimation is it takes somewhere between 6 and 10 dives on average get comfortable diving ones first drysuit (as with most things, for some it might take more or less).
Just like the semi-dry, the primary major consideration is how the suit fits. A good, proper, comfortable fit is so paramount that no other feature is worth considering if the fit is not correct. A drysuit that does not fit properly is not only a bane to dive, but can add to the danger that is already prevalent in diving.
All-in-all, there are a lot of choices available on the market. Do what works best for your budget, and the type of diving you plan to do most frequently, but if you are going to spend a lot of time diving in waters around 45F like your original post hinted, then I urge you to consider investing in a drysuit for the sheer comfort it provides....diving is just simply more enjoyable when you are comfortable.
Hope that helps.
-Z
Or just trying to get your post count up?
