A semi-dry suit is basically a beefed up wet suit.

It usually has a dry suit zipper and some type of seal at the wrists and ankles. Water still gets in, and it fits tight as a standard wet suit, but the seals and zipper keep the water from flowing around. They are somewhat warmer than a regular dry suit, and about double the price.

In my opinion dry is the way to go if you are in cold waters. For me anything that requires more than a 3mm wet suit and I am going dry. I would consider a 5mm wet suit, but I would never dive a farmer john and jacket style heavy wet suit again.

If you would like to try one out, most dive shops offer a dry suit class. They are usually under $100 for the class and includes the use of a dry suit. Many dive shops will even take the cost of the class off the purchase price of a dry suit if want to buy it from them after the class. It is nice because you not only get some training, but you can make your own decision about diving dry vs. wet.