We are thinking of booking the 7day Glacier Bay one way cruise from Vancouver to Seward.
Balconies are not available unless doing extended land cruise. So we need to choose cabin with window. Which is better, C on the lower promenade deck, or D, on the main deck below?

The lower promenade has the outside deck so you can go right out the door and be outside.
Main deck is one deck down and you have to go up one deck to get outside.

Lower Prom. is our pick

But on lower Promenade Deck other passengers will also be walking around. During the day they cannot look into the cabin, during the night you will to close the curtains.
I would opt for Main deck to have more privacy.

If you want privacy, the main deck is better. As I walked the promenade deck, I wondered how much of the conversational noise could be heard in the staterooms with windows on the deck. Some people were quite loud on their deck chairs. Also I didn't find the stairs to the upper deck annoying at all plus there is an elevator if you don't want to walk. Lower in the ship is usually also better for motion sickness if that might be an issue.

I would still go for Lower Prom. At night there is nothing to see anyway so it doesn't matter if the drapes are closed.

People in deck chairs are there in the day, at night they stack the chairs. So any noise would not disturb sleep just daylight time in the cabin.

We are early risers, so when they hose off the deck in early morning, it doesn't bother us but a Real light sleeper would hear it.

Whatever you choose will be fine - Alaska on a HAL ship - great combo.!!