I would say that a black cocktail dress would be fine. Some ladies get highly glammed up in long evening-dresses and some wear cocktail wear. They all looked lovely. Trouser suits were not in evidence on formal nights for ladies.
The real difference is visible in the male guests who have to wear "Black Tie" or a dark suit ( most opt for the tux or dinner jacket. )
Silversea do give a guide to "What to wear" on their web site but your planned wardrobe sounds good and should fall in line with their criteria. It is not as regimented as it sounds and I'm sure you will have a marvelous trip.
By the way if you are planning on dining out in Venice I would recommend the Trattoria Alla Madonna just near the Rialto - it is a restaurant used by the local inhabitants from the mayor and gondoliers to stars and visitors alike and is reasonably priced. It's down a funny little alley but it is worth finding ( it has a web site too so you can check it out ).

Have a super time

Raskelf.

Thanks for your suggestions. Will pack accordingly. If you any other suggestions for things to do in Venice, that would be great!

Assuming you haven't yet sailed here a some vague and meandering thoughts on things to do in Venice. Have coffee or hot chocolate at Florians in Piazza San Marco ( where Casanova paused for a cup after escaping from the Doges Palace dungeons ) - it will cost a small fortune but the experience of sitting in that square, sipping your drink while the little - but exquisite - orchestra play for you is a joy. Next....Get lost ! I'm not being abusive ! Honestly ! But you should get off the main thoroughfares, down little alleys , over tiny ancient bridges and find piazzas, churches and cafes hidden like little jewels in the byways. Buy little fresh wild strawberries in the Rialto market and taste the strawberry and cream flavour. And mainly....look, not just ahead and around but upwards.....and enjoy the most wonderful, beautiful city on earth.

Regards.