Hello Carl, how are you? Did you mean to ask about April 12th, or the actual night of the sinking? I am not sure about the air temperature, but the water temperature was said to be around 28 degrees fahrenheit on the night of the sinking, April 15, 1912. Of course, that is 4 degrees below freezing, but sea water freezes at a lower temperature due to the high concentration of salt and minerals in the water. If you were asking about the 12th, I am sure that someone here has some data regarding that. Hope this helps.
Kind regards,
Tad Fitch