First off, congrats. It's a great place. I haven't been to Culebra for about 7 years, but Playa Flaminco (sp?) was great when I was there. May be better for sand/surf than snorkeling, but beautiful either way. You can get there via taxi when you get off the ferry from the main island.
If you have time, check out Vieques as well. I was just there last winter and saw some pretty good stuff. Bio Bay is great and absolutely worth the time and money. Go with Tim at Aqua Frenzy- he and Eddie are both great. The mangrove tour is pretty good, too.
The best snorkeling on Vieques is in the area where the military used to exercise. We found a great section of fans, etc at the end of (what I believe is) Orchid or La Plata beach. Rent a car or some scooters, head out to Red and Blue beach (the locals can tell you how to get there), but keep on going to the other side of the peninsula that makes the far end of blue beach. You have to swim ~200 yards along the shore- into the bay, not along the peninsula- to get to the good stuff, but it was worth it for us.
Hope you like it.
Here's a map:
http://www.elenas-vieques.com/beaches.html