I don't think water as too "see" the words to be affected by its vibrational frequency. However any metal may or may not hinder it's efficacy. I'm basing this fact on biofeedback analysis where metal is known to disturb readings of emanated frequency.

You may want to avoid opaque materials for other reasons though, specially if made of plastic. You don't want the chemical to leach into your water.
Glass would be ideal as it is relatively inert; certainly exposing the water to sunrays can also be beneficial, another reason for glass containers. Colored glass containers can very good as they add "color therapy" effects into the water. It is known that colors affect moods and conversely the immune system the same way your water is affected by your emotions (it's all vibrational frequency after all).
Stable ceramic cups are also acceptable in my view.

You may want to focus your intention when writing and labeling the bottle as to imprint strong vibrational frequencies with the words. The same word can affect me and you differently so it's important to put your personal intention into it.

Mike

I have written "love" with a little heart beside it, "graditude" with a smilie face beside it, and "peace" with an upside down peace sigh beside it, (because I think that it the way it should be, like a tree, and like when you make a peace sign with your fingers).....I really feel it improved DRASTICALLY, and INSTANTLY my mood, and my toddlers mood, and the (used to be ) grumpy girls that lived downstairs....I also recently made a few orgonite pendants and "paper weights" with loving intentions that spread to the entire neighbourhood. :)