Which term would describe a vessel that is riding very low in the water?

The term that describes a vessel riding very low in the water is "awash." In nautical terminology, when a vessel is described as being awash, it means that its waterline is at or very near the surface of the water, and it may even have some of its deck or superstructure submerged. This condition usually indicates that the vessel is heavily loaded, which can affect its stability and seaworthiness.

"On the surface" generally refers to a vessel simply being afloat without any specific implication of how it is riding in the water. "Submerged" suggests that the vessel is mostly below the waterline, which is not accurate in this context since the vessel is still partially above water. The term "floating" typically indicates that a vessel is buoyant and remains above the water surface without specifying how low it rides. Therefore, "awash" is the most precise term to indicate a vessel that is riding low.