Pelican Reef is very nice. I haven't stayed there but have seen it.
Capricorn was really nice when we went. Granted, I was with a bunch of loud fishermen, but without them, it could have been a romantic place!
The new-ish owner and manager were also very attentive, and I would bet that they would do as much as they could to set the mood. And, the moonlit boat ride is nice, too.