Mauritius is particularly busy during December and temperatures can be high. Heavy rain showers are likely but help reduce humidity so are welcome in the afternoons when temperatures can be particularly hot. Water visibility is good at this time so Mauritius is great for diving too.
One of the Indian Ocean islands that is somewhat off-limits throughout December is Reunion. It is the rainy summer season in Reunion and subject to hurricanes so it is not the best month for exploring this stunning island.