I'll avoid the normal "Cali" correction and figure you are talking about the Winter.
While just about every major area in California(with the exception of the Central/Northern mountains) will be warmer than Wisconsin in the winter. If you want the warmest weather you will need to head to Southern California. And if you really want warm weather..think to other places such as Florida, Hawaii or places like the Caribbean or Bahamas.
The temperatures will average around the 60's-70's during the day, 40's-50's at night. There could be periods of rain and lower temperatures, as well as warmer temperatures if the conditions are right.
If you are thinking CA is some warm tropical paradise with crystal clear 80 degree water..you are on the wrong coast. During the winter our average water temperature is in the high 50's-low 60's. Unless you have a wet suit you are not going to want to spend any extended time in the ocean.
One other note, people who mention the desert have probably never really been to the desert. As while it can be hot in the summer, it can be equally cold during the winter. The days will often not go above the 70's and can still be pleasant during the day. At night, they can easily get down into 30's and below freezing.