I guess "things to see" depends on your interpretation of what things you're meaning. It also will really depend on how much they disturb the area. So basically if you're looking for the touristy stuff that Disney sinks as the "things to see" we don't know yet. I'd assume at some point they'll disney-fy it but I'm not sure when. They have made a pretty big deal about this island being focused more on the culture and natural areas of the island. So maybe they'll leave it untouched aside from ropes and nets.
Marine life will really depend on what's already there and how much they disturb the area. I.e. if they bring a lot of sand in and cover up some of the plant life then most likely the fish living there will move on and there won't be much to see for a while until the area recovers.
Then if you mean "things to see" like you're expecting tons of fish like other excursions its also important to keep in mind most of those other excursions like in Nassau and such they are baiting the fish in and my understanding is Disney does not do that.
I would assume the first 6+ months there won't be a ton going on just because no matter what some of the area is getting disturbed so I wouldn't expect large numbers of fish and such if you're going that soon. I'm sure there will be outside of the "lagoon" area to snorkel with real excursions so that will be different. My comment is specifically for the kind of DIY snorkeling you can do at Castaway where you rent the gear and go.