In short
Seas = local chop; swell = travelled longer-period energy; wave height ≈ significant height.
Seas are the short, local chop created when wind blows on the water nearby. Given enough time and fetch, that energy organises into longer swell.
Swell is the longer-period wave energy that has travelled away from where it was generated — the “rock in a pond” rings that keep going after the wind at your beach has changed.
Waves (in surf talk) are usually what you want to ride: organised swell that stands up on the reef or sandbank. Forecast “wave height” is typically significant wave height for that sea state — not every tiny ripple, and not always the single biggest set of the day.
Examples
- 6 ft swell, no wind, no seas: glassy cylinders — surfers grin.
- 6 ft swell, no wind, moderate seas: same swell energy, but chop on top ruins the face.
Seas can sit on top of swell and spoil a session even when the swell height looks perfect on paper.