Tested it with only origins-forge-1.19-1.5.0.4-fatjar and caelus-forge-1.19-3.0.0.3 in the mods folder. When picking Merling and entering water their vision blinks between something almost like blindness and normal, not-merling underwater vision. The player essentially turns black for a few seconds before changing back to normal, see below:

When you first enter the water

A couple seconds later, rinse and repeat.

I also have the same issue with the Merling Origin, plus Auqa Affinity does not work correctly.

The 1.19 is still in beta and doesn't fully function for all origins yet, Edwin is still fixing the bug right now