I'm not as tough as Bear...I don't care for a 550-cord bandoleer carrying a liter of water; it can get kind of uncomfortable. I like a decent shoulder strap and if you're using a pouch of some type (which I recommend), they usually have some type of belt/MOLLE attacment on the backside which can then be attached to your belt for more secure carrying.
I have the Cursader pouch. It's roomy, but I wanted some external MOLLE webbing for a couple of "smaller" pouches to carry some Polar Pure and whatnot. I was just screwing around and used an EMDOM 4X4 pouch (fits great with both the metal and plastic cups) and added a few pouches:
Here's the contents I added:
Now honestly, after a little hike, it's just too bulky. I need to rearrange, but if I'm carrying a canteen with cup, I still like to add water purification (typically Polar Pure), a little 550 cord, 100mph tape and maybe a cravat (can be used as a debris filter before adding your purification tablets/solution). Even with the size, I have an older USGI 2qt sling which works great and makes it much more comfortable to carry.
I like the Toadstool recommendation that Robert mentioned (now discontinued! But I found one on clearance

). I have a few other pouch options I'll go through. I will say that I really like the Crusader stove option, just wish my little Vargo alcohol stove fit in the center...it would be a great stove option.
ROCK6