No need to scuba here. The water is pretty shallow, and you can see all sorts of amazing sealife from the surface. We went with MileMark Tours (captained by Earl, crewed by Marcus) as part of our hotel package. The boat ride is short (~30 min), and then Marcus led us through one of the underwater trails. That's right, trails -- this is (as far as I know) the only totally underwater national park with underwater trails. We had some time to explore the reef on our own (tropical fish galore, moray eels, and Caribbean lobster). (Warning: to those who get sea sick, the boat anchors at sea for this. Spend the time in the water, not the boat, and you'll be fine.) After that, they took us ashore to explore the most beautiful beach I may have ever seen. The sand is soft and white, but the water is so perfect that I didn't bother spending time on the beach.
We only went for a half-day, but there are many full-day tours -- and it would be well worth it to pack a lunch and spend your day there. But be sure to bring everything you need; it's a national park, and all it has is nature -- no facilities except what's on the boat.