Swell attenuating device comprising a horizontal plate slightly immersed in the incident sea swell, this plate being held in position under the free surface of the water and presenting perpendicular upstream (12) and downstream (14) edges raised to a positive dimension above the free surface of the water, so that the incident sea swell cannot propagate freely over the plate, each of the upstream and downstream edges being extended at their base by a tab-shaped element (12A, 14A) of the same specific length, the assembly thus forming a symmetrically profiled structure in the form of a so-called "camel's back", device wherein one at least of the two elements formed by the perpendicular upstream edge and the plate part, or raft (10A), laid between the upstream and downstream edges comprises orifices (20) over part of its surface.