water level sensor wetting current
The item shown at the link below looks exactly like your picture. The spec lists the contact resistance as 10 mohms. I would guess you have a bad switch.

https://www.aliexpress.com/item/Level-switches-Liquid-level-sensor-Liquid-level-controller-Plastic-ball-float/32224577746.html?spm=2114.10010108.1000013.3.140c68d8zovlJx&amp;gps-id=pcDetailBottomMoreThisSeller&amp;scm=1007.13339.90158.0&amp;scm_id=1007.13339.90158.0&amp;scm-url=1007.13339.90158.0&amp;pvid=e8d5a85a-c807-4949-8658-7548639be20d