Here on the south coast there usually is more rain in December, but only as showers. The twoday/allday rain I remember from winter/autumn-Denmark I've only experienced once. And the showers usually don't start until after 2 in the afternoon.
It's very nice in the morning with clear skies and a slight breeze, so you should be able to get your part of the sunshine cake and don't get too charbroiled from it either.
If you insist on spending time on the beach in the afternoon, seek out a place near a bar and ride out the 5-15 minute showers under the roof holding your new love (Siboney Reserva Especial).
