Just avoid all the canal-side places near the Rialto. They serve pretty poor food, are over priced, and are aimed at the mass tourist market.

"Most Romantic" might mean that a really poor violinist scratches away at his fiddle in your loved one's ear while you are trying to whisper sweet nothings into it.

In Venice, the best, most intimate, meals are to be had in small places.
Don't on any account go to Harry's Bar.

I'd suggest that you post this question in the Europe board, with the tag "Italy". The travel tips section does not see too much attention.

You could try this joint (it's a Hilton, so likely to have a large contingent of Americans using frequent flyer points)

http://www.molinostuckyhilton.com/restaurants.php