Hi! Cristina may have mentioned that I get some support from Salomon so my knowledge is limited to their line of packs.
If your focus is carrying water, you might like the pack I bought last week and tested on the weekend. It is similar to other packs I've used for years with some improvements. When I do it up snugly, it is fairly non-bouncy, especially after I drink a little of the water.
S-LAB ADV SKIN 5SET
Like all the Salomon running packs, it comes with a bladder sleeve that is designed to hold their 1.5L bladder. You could probably fit in a different brand of bladder if it had a similar shape. I haven't yet tried it with this pack but with another Salomon pack, I have squeezed in a 2L bladder without using the bladder sleeve. (The sleeve provides a little insulation from your back, which is good for both temperature and comfort but I did a 29-hr race in June with a 2L bladder without the sleeve.)
It comes with two removable 500 ml soft flasks on the front, which is nice since you can just use them alone on shorter runs. You could also use them in addition to the bladder on the back. So... lots of water options.
Btw the usual way to use a Salomon running pack is to run the bladder hose under your arm and up the front toward your mouth, but I run the hose over the shoulder because that's what I'm used to. Either way works.
This pack has a few small pockets, zipped and unzipped, to hold snacks and other things. Behind the bladder pocket is another pocket that runs the length of the pack. It is open at the top but you can stuff things into it without much risk. I use an ultra-lightweight dry bag inside pockets like that to ensure that I don't lose anything and to keep things dry. There is room for a first aid kit, light jacket, toque, wallet, phone, etc.
There is also a big open pocket across the bottom back to stuff things into. I had my phone in there while I was podcasting on the weekend but I only felt secure about that because I would have known instantly if my phone had fallen out. It's a great place to stuff a jacket but I'll have to experiment some more to see what it is best used for. I may use another lightweight bag in there to ensure that nothing falls out. I have seen people in races with that outside pocket stuffed with food and it looks really secure but I'm the nervous type!
If you wanted to carry more, there is a 12L version of the Skin Pack with pockets configured a little differently.
There are also 10L and 15L Skin Pro packs for trail running that look more like a traditional backpack. Here's the 10L one.
And although it doesn't meet your needs, this is the pack I now use for most runs where I carry water and don't expect it to get too cold. I can carry two 500 ml soft flasks in front and on really hot days, I have put two additional 500 ml soft flasks in the large back pocket in a ziploc, just in case. I usually carry an ultra-lightweight extra jacket, a first aid kit, wallet and keys, phone, food, TP and other small items. This is super comfortable but there are times when a bladder is just easier than fiddling with multiple soft flasks, which aren't super easy to put into the pack if you're in the middle of a race!
S-LAB SENSE ULTRA SET
There are other ultrarunning pack brands you could look at but I don't have any experience with them.
Happy shopping! (I love outdoor gear shopping.)