Picking a beach in Mauritius is really about three things: which coast the wind is on, whether there is shade, and whether the reef break sits close enough to keep the water flat.
The west coast is calmest between November and April. The east is windier year round, which is why the kitesurfers are there and the families are not. The north is the most developed and the easiest to reach on public transport. The south is the wildest and the least swimmable.
A rule that holds most of the year
If the wind is up and you want flat water, go west. If you want space, go south and accept that you are there to look, not to swim.