If you do the shore snorkel route,
I love to go to Cliffs alongside the Blackrock Marriot.
Who else has good "walk-up" places to snorkel in Maui?
We were just on Maui and I was in a wheelchair, because my arms were too weak to work the crutches. My husband pushed me on the Kaanapali walking trail. There is a nice shady spot at the end of the sidewalk near Blackrock (coming from the South.) Depending on your comfort level of leaving your wife, she could comfortably sit in the shade while you walked over to Black Rock and snorkeled. It is a nice spot to rest and would be a great place to read a book.
I agreee with Denise on the Maui snorkeling tours. They frequently do not find a good place to snorkel, and will go out even if the waters are way too turbulent for an enjoyable trip. The boat are usually crowded. A tour to Molokini is nice if you want to see the feet of other snorkelers. Most advertise "Turtle Town" which seems to be an ever moving target where they basically pick a spot and stop. When we used Pacific Whale Foundation they determined, due to the water advisory that they couldn't go to Molokini they just chose another spot offshore and did not offer a refund, since they couldn't control that they couldn't go to Molokini (It was also a three hour tour of asking for donations, and rationed food and drink. I would never recommend them based on our experience.)
But if you definately want the boat trip I recommend Pride of Maui. We did our first snorkel tour with them and they were okay. They had an easy entry water stairway on the back of the boat, and have a covered lower deck. They have a glass bottom viewing portion on their boat, so you wife could see some fish too, and maybe even some feet.
The best snorkel tour is on the Big Island, on Fairwinds. Great boat, great crew, and the water is always calm where they snorkel. I do this on every trip to the Big Island. Maybe on your next trip....