I can give you a few ideas for Cristmas destinations with hot weather:
1- Arabian Gulf.
Places like Dubai or Abu Dhabi have warm weather during December and you can find a mix of cultures great hoteles and shopping centers and of course lovely beaches.
2- Canary Islands, Spain.
There is an average temperature of 24℃ in these beautiful islands so you can go for a swim in winter time. The place is also well known as a surfer's paradise.
3- The Caribbean.
Places like the coast of Mexico, Dominican Republic, Grenada or Jamaica have perfect weather and the most beautiful beaches.

These are just a few suggestions, you can check other destinations that are quite warm during Christmas time in Asia (Thailand or Bali), in America (Brazil and Argentina) or in Africa (South Africa).