Do almost all modern smartphones use piezoelectric speakers?
No, on the contrary, most if not all of them are common dynamic speakers, just small and usually employ an neodymium magnet.

Many (if not most today) have a slightly different configuration from this one in the image, with a larger square coil all around the diaphragm (in order to increase power density, having the biggest possible coil for that frame).
Piezoelectric speakers are limited to trebles, and even then their frequency response is extremely limited and usually far from ideal (from being flat) in the desired operation range.
They'd never be able to reproduce all the range a smartphone speaker can (even considering how poor they are).
The reason your speakers got damaged is most probably purely mechanical, water getting in the magnet and between the coils or behind the diaphragm will absolutely destroy the sound quality, and to make things worse it can be very stubborn to evaporate from there.
Not to mention it can also cause rusting of the metals inside, and if they rust they can swell and lock or friction against the coil.