Why do Boeing suggest landing parallel to the swells on the windward side of the swell
Landing into wind means touchdown is going to be somewhere between the crest and trough of the wave, and the height between the two can be considerable. Look at video of a ship and the degree to which it pitches when running into wind, trying to do the same at a touchdown speed of some 140 knots or so needs no imagination. Landing on the windward side means that you benefit from the updraft and the surface is going to be as level as you will find, and are removed from the downwind side of the crest which will be breaking if the wind is high enough. Since the wave set is moving it means that you will have a slight downwind component.
For a given wind the height of the wave is dependant upon the fetch. For a thirty knot wind the following fetch(nm)/wave height(ft) relationship applies
20/7.2
50/10.5
200/13.5