What is the main difference between a dry suit and a semi-dry suit? Why is there's such a big difference in price?
The main difference is the neck gasket. The neck gasket in a dry suit is made out of latex, and usually in a semi-dry suit is made out of a different material – let's say neoprene, or something else that will usually be a little more comfortable. It's usually not as watertight, hence being called a semi-dry suit.
To a lot of people a latex gasket is just fine, it will feel comfortable. But to many others it might be really uncomfortable. Having a latex gasket around your neck all day, or in some cases it can even cause allergic reactions. An alternative to latex is always a great thing to have. Don't get me wrong, there are alternatives by brands to latex that will be very dry. I'm not saying that latex is the only way to get a fully dry dry suit. There are other options, but usually when we're discussing dry versus semi-dry, if they say semi-dry that usually means that if you get dunked, if you go upside down, if you're swimming for a little while probably a dribble of water will get in through that gasket versus whatever material is used to be an absolutely dry dry suit.
I'll give you an example: when my wife and I started paddling – for either the cold months or the months where the water was still cold, we both went for full dry suits. Eventually, she couldn’t stand the neck gasket. It used to bug her all the time, so we ended up changing her dry suit for a semi-dry suit because we also realized that she was never really paddling whenever an occasion would warrant a full dry suit. She doesn't enjoy paddling in rough water, she doesn't enjoy paddling in rough conditions, she doesn't enjoy paddling in very, very cold conditions, so it makes her happy to be more comfortable in semi-dry suit which is perfect for the paddling conditions that she usually likes. On the other hand, when I go out I know I'm going to be dumped around a bunch or if we're surfing I know there's a very good chance I'll be upside down, or I'll get pulled out of my kayak, or if we're going to practice things like rescues, that latex gasket for me helps keep everything out and keeps me dry.