Average water temperatures for Scripps Pier in La Jolla (from National
Oceanographic Data Center):
  1st-15th November   63 F
  16th-30st November   61 F
  December   59 F
  January   58 F

Temperatures at depth (~100 fsw) in La Jolla Canyon or at Wreck Alley (Ruby
E and Yukon) would be in the 55 - 58 F range in December.

Visibility is variable.  15 feet is a reasonable prediction.  Storms can
churn things up and make it 5-10 feet for a couple of days.  December is not
the stormiest month in San Diego.  On the other hand, some of the best
visibility of the year (30 feet +) can occur when the winter weather is calm
and sunny.

You can check out San Diego dive reports from previous Decembers at
www.divebums.com