The Indian Ocean is VERY BIG and you need to be much more specific!
North Indian Ocean ruled by the NE &SW monsoons. NE good, SW mostly cloudy and wet at times.
Beware Spring & Autumn months, say May, June, September, October when wind drops out and you are in the cyclone season - rare but very nasty especially near Ceylon & Bay of Bengal.
Mauritius area monsoons much less apparent but cyclones more common and very nasty (La Reunion holds the World record for rainfall totals over 3, 4 and 5 days in excess of a metre!
I would say mid december to march. From mid april it starts to get very muggy and oppressive. Dec is very fresh, although early in the month it can still be a bit overcast. Always hot though.