NCL and Carnival both allow jeans in the dining room. Carnival has Elegant nights and on that night no jeans in the dining are allowed. However, both these lines are casual so don't stress you picked the right line for casual crusing.
Richstacy I would agree with you if they picked a line like Celebrity or HAL where the dress code is more upscale. To my way of thinking if you are adhereing to the rules of the cruiseline then other no cruisers have no right to tell you to pack dockers. I mean this in the nicest way. :-D
However, to the OP if you are coming from a cold climate you might find the jeans to warm for the Carribean heat. The dockers might be cooler and wear better in the warm weather. Just something to think about.