I've not been on an Ambassador cruise yet, but this suggests you will be okay everywhere bar the main dining room.
https://www.ambassadorcruiseline.com/faqs/
What is the dress code on board?
During the day, we suggest that you wear casual clothing for your comfort. For the evenings on board, every day the Daily Programme suggests, as a guide, a mode of dress for that particular evening’s events.
There are generally two ‘formal’ or gala nights planned every 7-12 nights when many gentlemen wear dinner jackets, although a lounge suit is quite acceptable. Ladies on these occasions have a chance to dress up and often opt for evening or cocktail dresses. Please note that jeans, T-Shirts, shorts and open neck shirts are not permitted in the main restaurant on Formal or Gala evenings.
On evenings proposed as ‘smart’, a suit or smart jacket and trousers, with or without a tie, for the men is suggested and the ladies have further opportunities to look elegant in cocktail dresses or trouser suits.
A ‘casual’ recommendation often covers evenings spent in port or when a special event such as a deck party is scheduled. In these cases, the choice of dress is left entirely to you.