research, research, research
There are articles on the topic of charge storage in water in the research literature. I'm not going to list any of them here because I'd have to first review them for application and (1) I'm not even sure exactly what your application is; and, (2) It would be a lot of work that you should be doing, not me. But you should make some attempts here to find and evaluate the possibilities.
My own mental models about the idea are probably deficient as my chemistry and related physics backgrounds are minimal. But I can make some general guesses about using a single electrode where it must be food-grade 304 steel (or maybe 316.)
My first choice to gain a leg up on understanding these things better (and this is left for you, not me) would be the 1973 edition of "The Chemistry of Electrode Processes" by Ilana Fried. Any attempt to change or alter the voltage on the probe may slightly alter the number of nearby OH- or H+ ions in proximity to the surface of the probe in water. (Technically, this is only true if there were a 2nd probe. But my ignorance allows the idea that the ambient environment surrounding the water can allow very small variations.) So with AC applied there may be some slight back-and-forth of charge motion. But I wouldn't expect to see a lot.
Bottom line is that you should perform your own research on the topic. You know your circumstances better than anyone else. You can interpret what you read and consider it in the context of your situation better than anyone else. You can consider changing/modifying/altering your initial ideas based on what you gain from this study better than anyone else.
So this is job #1 for you, right now. Do your research. Then come back here, inform us, and ask a better question based upon your new state of mind, thoughts, and research materials.
experimental suggestion
The above point made, I do have a single thought about an experiment that may help make a useful observation. However, like any experiment into the unknown, I cannot tell you that it will yield useful results. You may only find out that the idea produces nothing of use and only tells you that my idea is really just a dead-end. Even then, though, at least it would close off one avenue from further study and it would also help you better interpret the above research that you still need to do.
So, if you are game, get out a logbook and perform this experiment.
The idea here is to create a very fast edge and see what happens with a probe in water vs the same probe out of water. The following circuit can produce a very very fast edge:
Image may be NSFW.
Clik here to view.
simulate this circuit– Schematic created using CircuitLab
Make sure to use the 74AS14 here. It's a shockingly fast Schmitt trigger and will create very sharp edges.
Also, in construction, make absolutely sure to provide a bypass capacitor soldered as close as possible to the 74AS14 IC's power pins. That's important. The same goes for the resistors. These should be kept with very short leads to the actual probe itself. The goal here is to take as good of care as possible in the construction to avoid as much as possible in uncontrolled and/or unknown effects so that you can focus on whether or not anything useful is to be seen from this.
You can either use the first stage to create the oscillator or you can drive it with a 555 of some kind or you can drive it with an MCU board. That's your call.
A question is what to do with your ground. (Obvious question without two probes.) I may consider drawing out several different ideas in my logbook to try. Could just be a nearby wand adjacent to the water. Could be just a plate as the bottle top. Could be other things. But I'd write them down as a list of ideas I'd like to try in the logbook before starting out, listing them in my preferred order.
The next problem is to see if you can "see anything here." I'd recommend having a very good oscilloscope to monitor the response at the tip here.
expect negative results
I frankly don't think you will get good results because there's no managed ground reference to consider and I expect things to quickly find an equilibrium state that makes observation of something useful rather unlikely. (There aren't two probes here so how will the output actually do anything in the water?)
But wherever the water sits, it is present in a world around. And it's all I can come up with as an experiment that doesn't cost a lot of time or money to build, assuming you have a scope already.