non formal dinner
 

for the dining on the ship, will we need jackets for the evenings or sleeveless ? ( september)

Depending on where your table is, you may need sleeves/jackets if you feel the air conditioning (for warmth, not for formalities).

THANKS. I HAVE A SLEEVLESS DRESS THAT I'D LIKE TO WEAR.

The dining room will be A/C, so it is more practical to wear shirts with sleeves.

Sleevless shirts are frawned upon in any food establishment for health reason. No shoes, no shirts, no service. No one wants to see someone's hairy arm pit while they eat, even at the hot dog stand, never mind a cruise ship.

Consider a cruise ship as a resort/hotel. You can wear a tank top to the beach but would you wear a tank top to eat at your local restaurant?

You can wear a sleeveless dress any night. I bring a sweater or a shawl that I take with me at night because the dinig room, theater or wherever we are might be can chilly with the AC.
They do not want men to wear tank tops or sleeveless shirts in the MDR but it is fine for women.