Unfortunately, all of those can be very loud.
(They're also all pretty darn good.)
'Romantic' is actually a bit hard to find at Disney (unless you want to go broke at V&A, which is admittedly very romantic (and expensive)). Even if I ignore your seafood/steak request, I'm trying to think of places where you can hold quiet conversations, not hear other conversations, and not feel like you're on display when the resturaunt is fullish. Fultons, perhaps? Still some close together tables, but quieter and often forgotten, and they have decent enough shellfish. Monsieur Paul's window seats are pretty tucked away, but it's a rare shot to get one, and they don't specialize in either steak or seafood.
Of the ones you listed, there's a few back areas of Yachtsman that are quieter then others, and the dark wood and subdued colors can certainly make it seem more romantic then others. However, you could also be seated in the middle of the room tables all around you and loud conversation. Put a request for a quiet seats and note it's your honeymoon on your res, and you may get lucky. Or not.
Flying Fish is one of the signatures most likely to have children there, and while at first being seated in the side/back room seems like a nice romantic place, it quickly changes when you realize said back room picks up all conversation and amplifies it making it probably the loudest signature. It's one of my favorites, definitely, but it can be loud. Likewise, Boathouse is extremely loud: your table may be more private, but the volume can be crazy if it's crowded (you may have more luck with a romantic lunch here, as odd as it may sounds- less people are in DTD at lunch)! Narcoossee's is probably less loud, but also has tables very close together in some parts, and due to the firework views even if you get a romantic 'window' seat, you might have to deal with people standing right next to your table (or right in front of it, outside). Thus, while I am very fond of all three places, I would probably say Bluezoo is a bit quieter and romantic (though sadly only a bit), especially if you get the back room there (rare, but possible). For Bluezoo, going as soon as it opens tends to be a bit quieter: crowds pick up as MGM closes and people get tired of Epcot (it then slows down again before closing).
