If you decide Coral Reef...I thought the calamari appetizer we had was good as was the kid fish of the day. The mahi mahi was pretty bad for a restaurant...like a Lean Cuisine (in terms of portion and taste). I can eat a fish Lean Cuisine, so I ate it, but this is supposed to be their signature dish?
Anyhow, I wouldn't avoid it, but I would probably order differently if we went again. When I'm at Disney, I know that I'm generally going to get just average food at the non-signature places (I go in thinking Applebee's or Olive Garden and please if it's a little better)...so I pick based on ambiance and schedule (or maybe one delicious food item like dessert, lol). So my advice would be to go with where you want to eat.
A little off topic, but we ate at San Angel after hearing HORRIBLE reviews of it. We went at lunch, and I scoured online reviews to see what menu items were generally seen as at least okay. We ordered those items and had a very nice lunch.