Yes, I think even if they just want to watch, they'd enjoy it. You will be in shallow water, so you can also still get in the water and not snorkel or stay on the boat, either way, its great fun and something not to miss.
It also depends on why they don't snorkle - if they are freaked out by the proximity of big swimming fish, they will probably NOT want to be in the water surrounded by the rays. They are very friendly and quite affectionate, and they tend to swarm you a bit for food. There is no danger from them, but some people might find it intimidating and/or frightening to be in their midst. We thought it was awesome, but there were a few who stayed on the boat not in the least inclined to mingle with the beautiful rays. Too bad for them, it is something that should definitely be experienced!
The water at Stingray city is only about 4 feet deep, so it's not necessary to snorkle. However, the previous poster is quite correct - if you don't snorkle because of a fear of some sea creatures, you'll want to watch from the boat, as they tend to swim between your legs!