There are a handful of options that are reasonably secure to carry water on your climbing harness
What is the best way to carry water on a multipitch route? Below is an candid comparison of a handful of options I've personally used. We just launched a new solution that we really like: a minimal water bottle sleeve that you can drop in your favorite 1 L bottle. I first sewed these water bottle sleeves for my own El Cap ascents, and then used those prototypes extensively for a few seasons - simple and practical sometimes wins out over ultralight. But there are, of course, quite a few options, and the others still have their place.
These water carrying options all fit well within the paradigm of backpackless multipitch climbing. One of the main benefits of this method is that each climber's provisions are on their own harness, which means that they can accomplish some self-care, like eating and rehydrating while belaying. This is in contrast to having one backpack for the team, where you need to both be stopped to access it (or only the follower is able to do self-care).
Backpackless multipitching also means that the water bottle needs to hang securely from your harness! A 1 L water bottle dropping down a route on top of people can be a real hazard.
Here is a list of multipitch water carrying options, listed in order of ascending weight per 1 liter of water carried:
Gatorade Bottle (or other PET bottle) + 2mm Cord Tether + Micro Carabiner (80g/1L)
Pros:
- Very low cost
- Extremely lightweight
Cons:
- Finicky DIY install - Cord must be skinny to fit around the neck, but such small size it is not very robust.
- Cap has no tether
- Bottle hangs sideways
- The cord is dedicated to that specific water bottle and must be replaced over time
- Lid is not wide enough to clean inside
Fiji Bottle (or other PET bottle) + Avant Hydro Hauler (101g/1L)
Pros:
- Simple drop-in fitment
- Secure and redundant attachments
- Fabric sleeve adds durability to very thin PET bottles that wouldn't otherwise handle climbing abrasion
Cons:
- Cap has no tether
- Sleeve is an extra item to keep track of
- Lid is not wide enough to clean inside bottle
Hydropak Soft Flask + Micro Carabiner (115g/1L)
Pros:
- Packs down when empty
- Simple one-piece construction
Cons:
- Neck hang loop is not very robust
- Soft flasks can burst open during chimney climbing (a real deal breaker for many routes!)
- Soft flasks can be difficult to handle one-handed handling while multitasking mid belay
Nalgene + Tahr Strap + Small Carabiner (215g/1L)
Pros:
- Simple add-on
- Robust metal plate and bartacked webbing
Cons:
- Bottle hangs low and sideways
- Strap plate can thread off Nalgene during use
- Does not fit Ultralite HDPE Nalgenes
Nalgene + Avant Hydro Hauler (238g/1L)
Pros:
- Simple drop-in fitment (can swap out different bottles)
- Secure and redundant attachments
- Wide mouth for easy cleaning after electrolyte use
- Could use with the Ultralite HDPE Nalgene for 167g/1L (this is my favorite setup)
Cons:
- Sleeve is an extra item to keep track of
These weight comparisons are all given in weight to carry 1L of water, which I do think is a useful general size for a day out climbing. With a bit more planning, you can bring two different size water bottles for your team.
A half liter is nice for the lead climber so they have less weight while getting the rope up, but then the follower carries a larger one liter water bottle. You then swap bottles based on who is leading the pitch
