This was our first cruise and I knew there was a Captain's night but didnt' realize there were 2 formal nights. My finace went unprepared, he didn't bring a jacket. So those nights we ate in the buffet
But luckily we ate at Sabatini's the last night (which was a complimentary dinner because there was a group of us of about 30) and there I finally had my lobster. Woohoo! lol. Sabatini's was nice and I believe worth the $20 cover charge. You'd be just fine with the attire you mentioned.
Next time I know how really formal those nights get, so we'll be prepared. But I know from speaking to staff that on the second formal night, there is lobster tail in all the dining rooms.