I haven't cruised Carnival, but have cruised one of the 'upscale' sister lines... we love cruising, but as travelbug said, Carnival is probably not the best choice for 'romance'.
Also sailing out of Miami that might be more 'romantic'... Crystal, Celebrity, Costa, Oceania... there are more, but those would be my top 4 to consider, personally. Crystal & Oceania are generally smaller ships... so you'd feel more motion and there would be less to do on board (although most cruise lines offer a wide range of programs for passengers... karaoke, kitchen tours, cooking classes, wine tastings, etc. plus pool-side games and the ever-present bingo
)... Celebrity is very 'posh'... dress codes are taken more seriously... the clientle tends to be on the older end of the spectrum, but generally very few kids/young families... Costa is a sister line to Carnival, but again, tends to be smaller ships.
All that said... not all those lines will necessarily be cruising out of Miami when you want to go (for example, Crystal "winters" in Miami... so they only sail from there in December & January)... a number of the smaller ships avoid the Caribbean in the June-November hurricane season.
Also consider that a number of the lines that cruise out of Port Everglades offer 'transfers' from Miami International Airport, too, though... which opens up more options, too.
I strongly recommend, if you're thinking of a cruise, that you check out the forums on cruisecritic.com ... there's a wealth of info there on different lines, different ships, ports of call, excursions... it's amazing!