I will be on the Diamond next week, 12-19 July in the Med. I am travelling with my 14 year old son.
I am planning to wear dressy sundresses (long) and skirts with sweaters or knit tops for dinner every night. Just try to get a 14 year-old in a tux. Laughable. He bought a totally polyester "Chinese Tux" in Chinatown in NYC last month - He's going with that for "formal night". The other nights I will be lucky to get him in a sport coat/tie. I figure there's always room service for him as an option!
Absolutely EVERYONE I've talked to about dressing on a cruise says the cruise line always tries really hard to suggest that people dress up. Hardly anyone does - can you imagine packing and carrying a different outfit for dinner every night? It's just not practical - or necessary. I don't even own a "cocktail dress" or a "pantsuit" - nor do I have any desire to wear either. I will be presentable and comfortable - and other than that... well, everyone will just have to deal with it.