While this is a great location, there are a few other places which are quieter, just as nice (if not better) and have better snorkeling within a few metres of the lagoon.
The first is Aroa Beach. Located near the Rarotongan on the South-Western part of the island, this is a marine reserve. There is an abundance of fish-life, great for snorkel enthusiasts. The beach itself is also quite nice for relaxing and sunbathing, with far less wind than what is frequented in Muri Beach. If you choose to take a trip to this area (I highly recommend you do), please try not to stand on any coral as it takes years for coral to regrow, and less coral means less sealife for your family and others to enjoy.
Visit https://www.tripadvisor.com/Attraction_Review-g309713-d3218299-Reviews-or10-Aroa_Marine_Reserve-Rarotonga_Southern_Cook_Islands.html for great images by visitors who’ve already discovered this paradise gem.
The second is Black Rock located on the Western side of Rarotonga, only a short drive past the airport. This area is by far the most underutilized beach on the island. On days when the southern part of the island is windy, Black Rock is often sheltered from the wind. This makes it the perfect spot for relaxing on the beach (to the right of the actual Black Rocks), and for swimming and snorkeling. During the evening, it can also be a perfect backdrop for keen photographers wanting to capture a nice and unique sunset shot with the Black Rocks in the foreground.
Visit https://www.tripadvisor.com/Attraction_Review-g309713-d3215853-Reviews-Black_Rock_Beach_and_Nikao_Social_Centre-Rarotonga_Southern_Cook_Islands.html for images of Black Rock and the stunning sunset photo opportunity that I mentioned above.