Drainage systems for excess body fluids and associated methods are disclosed herein. A drainage system in accordance with an embodiment of the present technology can include, for example, a drainage catheter, a first reference line, a second reference line, and a pressure sensor assembly. The catheter can include a flexible interface member and an inlet can be placed in fluid communication with a site of excess body fluid within a patient. A first flexible region of the first reference line can be in pressure communication with the flexible interface member, and a second flexible region of the second reference line can be in pressure communication with the surrounding atmosphere. The pressure sensor assembly can be spaced apart from the flexible regions, and measure the pressures of the first and second reference lines. This information can be used to determine the pressure at the site of excess body fluid.