I travel to Mauritius every year to visit my family.
During my last visit I went on the swimming with dolphins trip.
It was a great experience, and I highly recommend it.
I took a full day trip, leaving from Black river, including a swim with dolphins, snorkeling and including also a lunch and visit to one of the island (I don’t remember the name of the island).
I ordered this trip and also 2 more activities (catamaran cruise to Gabriel island and diving trip) online just before arriving to Mauritius. I suggest that you will check this website: Mauritius Attractions
, I know they offer many activities, and from what I checked, their prices are very good and I very much enjoyed all the trips.
Good luck, I hope this helps. Wishing you a good vacation in Mauritius.