The idea is straightforward and slightly obvious in hindsight: a waterproof Bluetooth speaker already has a sealed, flat, driven surface, which is most of what a water cymatics rig needs. Picarsic demonstrates two different models, both linked in the description, and shows what has to be adapted on each.
The honest limitation is control. You are stuck with whatever the speaker's amplifier and driver do at low frequency, and consumer units roll off exactly where the interesting Faraday modes live, so the accessible frequency range is narrower than a dedicated driver gives you. He does not oversell this.
As a first experiment it is hard to beat for effort spent, and it is the right way to find out whether you care enough about water cymatics to build something properly. Ten minutes, presented as a how-to.