Some CO monitors are cross sensitive to hydrogen. It would be very strange if anything immersed in seawater generated CO, but not a surprise at all that it generated a bit of H2.

Hydrogen is not a product produced that is normally associated with an anode, as hydrogen generation is normally associated with the protected structure (cathode). The Anode is more associated with the production of oxygen and oxides. In high current CP applications it is also possible to produce Chlorine gas from the anode.

I cannot think of n reason for CO to e produced under normal seawater Sacrificial CP circumstance. I would suggest that as suggested, the Carbon Monoxide meter is being hoodwinked by the oxygen and oxide production from the anode. Chemically there is quite a bit going during CP current being applied, but CO is not normally 1 of them.

Tqvm all for the feedback..fully appreciate it.

If there's an anode in a structure containing seawater, something else is being protected from corrosion- and the presence of traces of H2 in the headspace of the same structure would not be a surprise. The likelihood that this CO detector is seeing CO is diminishingly small unless the OP is not telling us the whole story. An oxidant will not set off a CO detector generally- CO is a reductant, as is H2, and the electrochemical sensors use this property to detect the gas. The difference is that you're concerned about H2 perhaps when its concentration gets near 10% LEL or 4,000 ppm, whereas 20 ppm CO is definitely reason for concern. The meter is therefore sensitive, and generally used in environments where CO would be present but H2 likely wouldn't be- i.e. around combustion equipment.