How can I find a tag under about 20 centimeters of liquid?
Passive high-frequency (HF) tags operate at 13.56 MHz and use indicative coupling to communicate with a reader. Thus, they are able to be read through water. It should be possible to interrogate a passive HF tag under 20 centimeters of liquid, but you would need to perform some testing to find out. The size of the tag antenna, the salinity of the water and other factors could influence your ability to read tags consistently.
—Mark Roberti, Founder and Editor, RFID Journal