Hmmm, snorkeling.... I'm mostly a diver, but I have snorkeled Fiji too. I would strongly suggest you try diving there (get certified beforehand if possible) - just so you can see the clownfish up-close in the anemones. It's difficult to appreciate all the little critters when you're a few or too many feet above the reef. But, if you decide to just snorkel, here are my thoughts:
There are truly great snorkeling places all over Fiji. I'll organize my answer by geography:
1. Are you interested in soft corals, hard corals, or both? In general, the further east you go, the more soft corals you will see. The Tongan Trench, which feeds the soft corals with nutrients, is east of Fiji.
2. Are you interested in rain forests or dry conditions? In general, the further east (specifically north-east) you go, the wetter and greener it gets.
3. Are you interested in great white/soft-orange sand beaches? Look at the Yasawas.
4. Are you interested in seeing waterfalls? Look at Taveuni (Lavena Coast).
5. Are you interested in dropping off the face of the planet? Look at Kadavu.
6. Are you interested in large resorts or small resorts? If you're looking for a large resort, stick to the Coral Coast on Viti Levu. If you're looking for small resorts, look everywhere else.
7. Are you looking for very shallow reefs (reefs that are exposed at low-tide)? Stay on Viti Levu (the main island). The reefs cannot be snorkeled when exposed.
Fiji, as you're probably discovering ranges from hostel-type accommodations to very expensive with not much in between. Not all resorts have snorkeling right in front of their bures, but the resorts offer boat rides to reefs off-shore. Unlike Hawaii, the water in Fiji is as flat as a lake so you need not worry about waves cresting over your snorkel. There are very few places in Fiji where waves actually crash on shore. Most surfing is done (which is "most excellent" by the way) is done way-off shore.
As for my preferences... difficult to choose. I truly love the Astrolabe (not accessible from shore - only by boat), but your accommodation expectations must be lowered. Beqa Lagoon also has some great spots, but you're in the middle of a large lagoon. The Mamanucas are Aussie-party-central islands with great hard corals reefs just off shore. Vanua Levu has good snorkeling, but the water is a little murky (runoff from shore). And of course Taveuni, Qamea, Matangi are great with the addition of soft corals.
I would suggest trying the Lau group - with beautiful atolls - but there are no accommodations on these islands.
I notice you're in WA. My suggestion is to fly Alaska down to LAX. The Alaska terminal is right next to the international terminal. Avoid a lay-over in San Francisco at all cost (due to weather delays). As for airfare, AirPacific was running some great deals this summer. Try cross-referencing with SkyAuction. If you're an American Airlines advantage member or Alaska member, you can try redeeming miles for a free flight on Air Pacific.
Do not stay in the Nadi area (where the international airport is located) or the Denauru (where the Sheraton is located) area. Nadi is adjacent to a bay (with no coral) and Denauru is on reclaimed mangrove swamp land (thus it is gray muck).
I hope this helps. I've been to Fiji many times and the country is a truly fantastic vacation destination.
Cheers,
Scott