I am considering the AF Air Scrubber. I wanted to clarify – is this watertight? Can the base sit in the sump while connected to the skimmer?

No, AF Air Scrubber sump placement is not recommended. The device is designed to filter air, not to be submerged or placed in water. Its base includes special moisture-absorbing components and ventilation openings that must remain dry to function correctly. Placing it in the sump would allow water to enter the unit and damage its internal structure. For optimal performance, it should be installed outside the sump in a dry, well-ventilated area.