I just can't like the idea of jeans or shorts in the dining room for dinner.
First, let me say that at the moment, I don't own a pair of jeans and haven't for many years. That said, I have noticed that jeans have become very accepted slacks for men and women. I'm not talking about faded, acid washed or tattered jeans. I'm talking really about denim slacks, usually in a dark shade. I've seen them dressed up with a sports jacket and loafers for men, or a beautiful blouse and blazer with high heels for women, and I don't think they're so bad. I'd rather see folks in nice jeans than old, baggy or worn out trousers.
I've seen men and women wear jeans in the dining room on "smart casual" nights on several cruise lines, some that would surprise you. I agree that beach wear is not appropriate in the evening, but I can't complain about a nice pair of jeans.