Device for studying and/or measuring hydraulic phenomena in non-stable media, particularly for studying sea swell. It is of the type comprising a post (10) immersed in the medium to be studied (A) and provided on the one hand with an upper float (20) of which a portion (22) must remain immersed regardless of the agitation state of the medium (A) and on the other hand with a lower ballast (30). The post is connected to anchoring means (80-81-82-83) by a flexible element such as a cable (73). It is characterized in that the ballast (30) is topped with a hollow body (12) which has an outer surface equivalent to the portion (22) of the float (20) which has to remain immersed and to an assembly (50). The hollow body (12) is drilled with holes (13). The post (10) receives at least two lateral stabilizers (60) which are superimposed and arranged on either side of the heave center z of the device. At least one measuring apparatus (52) is arranged inside the assembly (50) provided at a level where it remains immersed regardless of the agitation degree of the medium (A) being studied.