Those heading to Malaga during December should be rewarded with plenty of sunshine. However, even the Costa del Sol has some poor days during early winter.
The sun can shine on many days during December, and an afternoon stroll along the sea front in Malaga will feel quite pleasant as temperatures rise to the mid to high teens Celsius (59-66F).
Of course, the days are fairly short, and a chill usually sets in and hour or two before sunset.
The nights can be quite cool as well, especially in the more urban areas where dawn temperatures may typically fall to about 5 Celsius.

Sea temperatures are still surprisingly mild in December, and the more adventurous that take to the water during the month will be reminded of bathing off the Brittany coast,  or in the North Sea,  in early summer . Not pleasant, but a temperature tolerated by many thousands of tourists to northern european beaches every summer.
The mildness of the sea can still generate vigorous showers if cold, unstable air passes across it, but the relative weakness of the sun’s heating reduces the potential for inland shower development across Andalucia.

As low pressure tracks northeast across the Atlantic towards Norway in a typical December, so southwesterly winds flood northwest Europe for much of the time.
Further south, high pressure usually prevails and keeps the weather settled apart from the odd weakening front producing cloud bands and perhaps an hour or two of steady rain.
In some years, but perhaps a distant memory now, northern Europe can be cold and snowey as weather systems track eastwards much further south.
Active fronts bring copious amounts of rain to Iberia, especially to mountainous areas of the north, and even in the normally dry south successive bands of steady rain top up the reservoirs.

Fog is not common in December, although the odd 2 or 3 hours of rural  fog may disrupt air travel in the hours of darkness,  this most likely after a spell of wet weather.
A cold interlude is a rarity in December, but the Sierra de Mijas with snow on the top has been observed during December with rain and drizzle in Malaga, along with a brisk northerly wind, holding temperatures near 9 Celsius (48F).
The strong northwesterly wind usually occurs at some time during December with low humidity but temperatures exceeding 20 Celsius (68F).

Normally, though, there is that characteristic sunshine from a blue sky that makes the Mediterranean coast the joy-bringer that it always will be.