it uses IR (non-dispersive infrared (NDIR)) to read the CO2 spectrum. It lights up when plugged in. It has 2 air ports for air movement. I think the issue when I dunked it in water, the breather bag collapsed to plug up the air ports. I'm building a proper plastic box for it now with a breather bag membrane. I'll use rubber bands to hold the bag on. I'll have to find something heavy to glue to the box so it would sink.
From looking at the ppm in the air, it shifts from 400ppm - 500ppm. I'm thinking it'll shift back and forth as well in water. So I'll plan to take the average reading within a certain time frame like 10 seconds or so.