I agree with Sandy 100%----make sure that your travel agent requests that table for two for you. Have them flag your reservation as being a 20th anniversary.
Then, once you are on the ship, go to the dining room and check out your table assignment, which will be on your cruise card.
See the Maitre 'd that afternoon of embarkation if the table is not to your liking. Let him know that it's your anniversary. If he is able to do what you want, make sure to tip him that evening.
Happy Anniversary and Bon Voyage!