motion sensor - waterproof

Hello everyone, I'm quite new to arduino. I was wondering if anyone could suggest a waterproof motion sensor.
Let's say I have a glass of water, I would to put like a sensor, or metal sensor around the wall of the glass.
Any movement of the water would be detected by the sensor. Is there any sensor in particular that does this?

So the desired end result is to sense movement of the water? What kind of movement? Will the level of the water change? Is this supposed to respond to level changes, or should it ideally ignore level changes while only responding to sloshing? What about vibration? Should it respond to vibration, or ignore it?

How sensitive to motion, whichever type of motion you are wanting to sense, should it be?

One can detect movement with an accelerometer and it shouldn't be too difficult to make one waterproof.

It will only detect movement of the accelerometer though.
Stirring the water while the sensor is attached to a stationary glass won't give results.

Something you could do as well is to build a mechanical movement sensor.
If you screw a screw in a little spring and place that in a metal tube, you can use it as temporary switch if you move it hard enough.

Waterproofing electronics shouldn't be that hard using epoxy/resins, even hot glue or a few thick layers of waterproof paint may be good enough..

There are also piezo motion sensors, Sparkfun has some. I made some simple motion/sound sensors for Tribbles by merely mounting a hard piezo transducer loosely on its wires, so that it could bounce around a little if the Tribble was moved. So when it banged a bit on the nearby plastic structure, it generated electrical pulses. It would also detect loud noises. A piezo buzzer element can act as a speaker or microphone.

I want to make a fun and simple project, which involves sonification.

Basically, I'll have a glass of water, and when I put a tea bag in it, you can see the movement of tea dissolving into the water, that will create some pleasant harmonies.
I want to detect the motion of the tea as it is dissolving in the water.
What type of sensors should I use?
I'm guessing I could sonify it in max msp, and for the motion I could do it in arduino and have those two to talk to each other?