Well you never know you really need to see what the weather is like as Roman said.
We booked a tour in the south east (Blue Bay) in November 2008 and the trip brought us up to Ile aux Cerfs. Northways everything was nice, with snorkeling, etc. but then after leaving the Ile back home we got some windy, rainy weather. Even inside the lagoon it was no pleasure.
Maybe you can keep an eye on the page below, there you have all the necessary forecast information for Mauritius. But keep in mind, when it's raining in one place, in can be perfectly sunny and warm just 5m away http://www.info-mauritius.com/weather-w ... recast.htm