Actually this isn’t the case with 13.56MHz passive inductively coupled NFC… the skin’s conductivity is not an issue because it has very low magnetic saturability… magnetic fields aren’t too bothered by tissue… this is why MRIs work and NFC still works even under the skin… what doesn’t work about an NFC sticker is that the skin sloughs off so fast that the maximum time you can get is about 7 days. There were even “NFC Tattoo” stickers sold once that had a special med glue that might last you up to 14 days but 7 was the average.
When it comes to skin/water interfering with RFID look no further than UHF backscatter tags… those use the electric field to transmit and reflect energy, and those get beat down like (insert your favorite punching bag here) and go from amazing ranges like 10 meters down to a few inches at best… some just stop working entirely.