The support (1) includes a working deck (2) and floatation members (3) supporting this deck (2). According to the invention, the spacing between the vertical axes (Z) passing through the centre of the floatation members (3) is such that the sum of the moments, taken in relation to the horizontal axis passing through the centre of the support (1) is perpendicular to the swell direction, of the vertical excitation forces of the swell on the floatation members (3) situated on one side of the vertical plane (P) passing through this horizontal axis is equal to the corresponding sum associated with the floatation members (3) situated on the other side of this plane, when the swell period is equal to the period of a swell having this direction and of which the yearly probability of being encountered on the site or the support where it is installed is 1/100.