So the Minecraft wiki says:
In Java Edition, drowned spawn individually inside flowing water or source water that is 2 blocks or taller (this can include waterlogged blocks). In ocean biomes, drowned spawn at Y < 58, or at least 6 blocks below sea level; this restriction does not apply to rivers.
However, I'm not sure on a couple things:
Can they spawn "floating" in water, or do they always spawn touching a ground block?
Can they spawn inside bubble columns?
Is the area where they spawn always "constant" (e.g. making the area of water smaller will just make them spawn more concentrated) or is it dependent on available water (making the area of water smaller will result in less spawns)?