You also don't want to pee in a semi-dry suit.
The true wet suit means that your entire body will get wet. Once the water has infiltrated the material, you body warms it and maintains warmth with that cushion of warmed water. However, there is still heat exchange between your body and the water you're swimming in. Thus, you will lose body heat.
The dry-suit insulates your body with air.
The semi-dry attempts to achieve a medium between. There is still some wetness, but not as much as a wetsuit.