Sorry, can't say anything about the sea urchins. On our day at l'Ile aux Cerfs (where Le Touessrok has a beach as well), we didn't see any. The water is crystal clear. I believe they are to be found rather near rocks.
As for the weather in May on the East coast, it will most likely be windy at times at not terribly hot, especially not in the evenings. May-October is considered 'winter' in Mauritius.
I suggest you have a look at the forum's weather page (look for East coast):
http://www.info-mauritius.com/english/M ... limate.htm