I had a very high quality 7mm fullsuit with a 5mm hooded shorty (Waterproof brand, semidry) that I happily used at 3-4C, from May to late October. It was wonderously warm underwater, and if I always had a warm area in which to get dressed and disrobe, I could have happlily used it for 80% of my diving even now. It's the part about being above the water that's the dealbreaker. Even in summer sitting on a boat with water next to your skin and a breeze, even under warm sunlight is amazingly chilling. In even slightly chilly conditions, the only way to stay warm after a dive is to get dry, which means you have to get nearly naked -and that's cold. Better yet, every time you take off the wet wetsuit and try to put on something dry you have a huge challenge to keep the dry stuff dry, Usually you very quckly have wet stuff and moist stuff, all of which is cold, as are you.
So I sold the wetsuit. I loved it, but I sold it to scratch together the money for something warmer. And the best part was, as soon as I had a drysuit my dive season was January 1 to December 31.