The Mauritius weather during summer is very mild. The climate is very temperate. Temperatures range from a minimum in January of 13 C (55F) to 26 - 27 C (79F) in July, August and September.
For fishing I recommend a light jacket, shorts and/or slacks, as it can get a little cool particularly in the evening. Ashore the same gear is generally in order as long as you look decent. A jacket and shirt with tie should also be packed. For the ladies some more dress up clothing might be in order. You may never use them, but some places do like a bit of formality. It will however rarely, if ever rain during the summer months.
Polaroid sunglasses or clipons are a must. With them you may see a Marlin behind the lures as well as the crew (which will take a while!).