For my entire life my to-go way of travelling was cabing luggage (some cheap, four-wheel suitcase) plus a small/medium backpack. Recently, I've been looking into the idea of using a single bag - mainly since a Patagonie Duffel would be much larger than my cabing suitcase (for major European airlines, the 55L model basically meets requirements, just tiny too long (57 cm vs. 55 cm). The decision is now further sped up by the fact that I sprained my ankle and need to travel soon.

Nonetheless, I still have some questions about travelling with single bag. I do not mind putting most of the stuff I usually carry in backpack to the duffel (think book, laptop, medications, charging cables, jacket/sweater). What gives me more headache is water bottle - I carry it in the bottle holders of my backpack, that Patagonia Duffel lacks. How do you go about it? Furthmore, for important documentation and items (ID, passport, wallet etc.) - where do you carry those (if you want to avoid holding it all in pants pockets)? Do you use waist bag or some other alternative?

TL;DR - when travelling with single duffel bag, where do you store (1) water bottle (700 ml), (2) documents and (3) small and important items? Thanks!!