A steady swell is a rare thing for most sailors. We spend most of our time in confused seas, the result of variable winds, diffraction affects from shorelines and bottoms, tidal currents, and wakes. Still, when working through the anchor testing series it was sometimes necessary to predict what I would find at a certain place and time, so I could be there and test what I wanted, when I wanted.
For example, a steady 40-knot squall lasting 30 minutes (fetch is the distance waves could have traveled during the squall, or about 4 miles) will generate steep waves of about 3 feet with a period of about 1.7 seconds. Those same waves generated by a 15 knot wind over a long time would have a period of 6.5 seconds and would be far more pleasant, and 40 knot winds given a long blow will generate mountainous waves I hope never to see; the largest I've sailed in were about 10 feet, and that was enough. I think I could go the rest of my life without winds over 20 knots.