Snorkeling companies in Maui

If you're just looking for a cost-effective way to introduce your family to the sport then consider renting gear from the venerable "Snorkel Bob's" - a Hawaii institution that offers surprisingly good quality equipment at reasonable prices. They have weekly packages too if you plan to be there that long. They (and the other dive companies) have a nice map of the popular snorkeling spots around the island and can recommend quiet beaches (ie not too much surf) where your family can get familiar with the gear before venturing out into deeper water.
If you're already experienced divers and want to book one of the boat trips out to Molokini Crater then a simple google search will show a long list of options, prices, and user ratings.

Hi Lorrie, I lived on Maui for several years. The main difference between the different snorkeling companies is what kind of boat you are on. The fish look the same no matter which boat you choose! The most popular option was a catamaran and the most popular company was Trilogy. They go out to the very popular snorkeling spot Molokini.
It also would depend where on the island you are staying. If you are in the Kaanapali area, check out the Gemini, which departs from the beach in front of the Westin hotel.

Before you book an excursion, consider where you are staying. At our place in Kihei, just across from Kamaole Beach Park II, you can do very nice snorkeling right from the beach.

I second the suggestion of Trilogy. We have sailed with them twice - once to Molokini and once to Lanai. They are an excellent company. I have heard, however, that Molokini Crater has gotten really crowded. Does anyone know if they are controlling the number and size of boats there?

and if staying at Ka'anapali beach, you can snorkel right off the beach at the Black Rock area. but pay attention to surf conditions.

I second Snorkel Bob's for gear. Snorkel Bob's is into reef awareness and conservation. They usually give you a free boogie board for a week with each snorkel rented, too. Gear is top notch.

Snorkel Bob's will also give you a map of the entire island. There are great snorkeling areas that don't require a zodiac or catamaran trip. In fact, we'll be snorkeling and a catamaran or zodiac shows up! I swore off catamaran's and zodiacs--we ended up going to beaches we already had been via car. The weather or surf was always too bad to get to the crater. Many people on the catamaran are there to sight see, so I've had trips cut short due to people getting sea sick and we never made it out to the advertised snorkel area.

The Pacific Whale Foundation runs snorkling tours (no whales during the summer). Maalaea Village near Kihei.
https://www.pacificwhale.org/cruises/maui-snorkel/

We have used all the "popular" companies and with all the controversy still prefer PWF.