The word "bathym" typically refers to the depth of a body of water. If you're looking for synonyms for "bathym," there are plenty of options depending on the context. You could use "depth" to refer to the distance between the surface and the bottom of a body of water. "Deepness" is also a good option for describing the relatively low level of light that penetrates water at greater depths. Other synonyms include "profoundness," "plunge," and "abyssal." It's worth noting that each of these words carries slightly different connotations, so it's important to choose the one that works best for your particular situation.