well, there are certainly more pretty beaches in mauritius than the one at coin de mire. sometimes, depending on the tide, it is a little narrow. you can swin off fom there, no problem to go out between the small fishing boats, but there are some stones and in the water it is a little muddy. but if you go left or right some minutes, you will reach small places where it is nice and sandy to swim off. or, for example, pereybère beach is about 20/30min to walk, 5min by bus, there you will find a small but nice beach.
i was told that the catamarans you see on the pictures belong to an operator who offers trips by catamaran. but they don't go out every day, maybe one or two of them leave once or twice a week. the small fishing boats belong to fishermen. i've never seen someone else going out with them.
there is a small boat and a glass bottom boat belonging to coin de mire. the glass bottom boat usually goes out in the afternoon for a snorkelling trip. but even if you just want to go for a swim or want to stay on the boat you can join the trip. it is included in the rate you pay for the room.
if you want to go out by the small boat of the hotel, ask the guys who work at the coin de mire boathouse. if it is possible, if they have time, they take out to swim or to snorkel to other places than those where they go by the official trip - but you must pay, it's not included.
Moreover, you can take the bus in front of the hotel and go to some other beautiful and sandy beaches. It will take you only a few minutes and the bus is quite cheap.