I've found the lower decks, the one below the lifeboats to be mostly deserted and thus very useful. But of course the view is somewhat limited. Higher is better generally, but sometimes the lowest deck gives you a different viewpoint. I've done both.
If you are there to shoot try and think like a photographer. Get into "nat geo" mode rather than tourist mode. Which means you might not get to enjoy the view so only do this if you are really there to shoot not enjoy. Get your head focused on shooting, scout the best locations along the rail, which ones have obstructions, how can you get from one side to other fast if you have too, what do you want to shoot. Pick two, three or four alternate locations. Do the primary first then hit the rest if you have time. Shoot high first then run down five decks and shoot again from a different perspective. Do your research, talk to the staff, often there are on board photographers that you can chat up.
I don't remember there being too big a crush on getting a rail space in Glacier bay but when I was there it was quite cold so most people took a look and went back inside.
The ship did rotate so if you are patient everybody gets a look.
I don't think eating will be an issue I do not remember us being there that long. Bathroom breaks might be an issue though

Take a snack bag with you, granola bar or candy bar. And dress warm, you can always shed a coat if you have to.