The open ocean can occasionally generate swells that measure 20m between peak and trough. A wave of this size would have a distance between peaks of around 600m. Large swells aren't very steep. The cross sectional area of one of the peaks would be around 2000m^2 and a 1m slice of a peak would therefore weigh in at 2000tons. And this slice would contain around 44kWh in potential energy. And since one of these waves arrives every 20s that means the average power required to flatten a 1m slice if the wave front is 7.85MW. That's a lot of power. It's about the output of a large wind turbine.

The best method for suppressing a swell of this size is probably with a wedge. A swell becomes a wave when it reaches shallower waters and is wedged up. The bottom part of the sine wave is hits the bottom and bounces up making the top part of the sine wave bigger and bigger until it becomes too steep to remain stable and the wave "breaks". A large semisubmersible platform could do the opposite by gradually forcing the top of the sine wave downwards.

Testing this idea would be fairly easy since semisubmersibles float at an adjustable height so you could take 10 defunct platforms and line them up at progressively lower heights and gradually increase the steepness of the slope they form until the movement is too noticeable inside the platforms to be comfortable. At this point you can measure how much you have attenuated the swell and can calculate from that how many more platforms would be required to attenuate the largest of swells.

The attenuation could be increased by making the columns of the semi-submerssible the shape of a stack of hourglasses. Since the water molecules in a wave only actually move up and down it provides the opportunity to make the molecules fight against themselves with such a profile. This concept is related to the idea behind the Tesla valve. The difference is that with vertical columns we can take advantage of the fact that water will accelerate as it climbs the thick part of the hourglass which will cause the water surface to develop a slope which eventually becomes steep enough for the surface to "break" like a wave. And this broken water is pulled down by gravity. Thus we have achieved the flow reversal that the tesla valve relies on without needing to explicitly guide the water all the way back around. Hence it's more cost effective than it otherwise would be. On the downwards portion of the sine wave though there is no such hack available to us so if we want to dampen the wave as a recedes we would need to implement full flow reversing channels like in a tesla valve. Though with the biological fouling that takes place on all ocean craft it might not be worth getting to clever about it since it would require constant cleaning.