Swell is the wave energy that reaches you from weather that is nowhere near you, and it usually makes for the most comfortable big water a boater will ever run in. It is generated by distant storms and wind systems, then travels for hundreds or thousands of miles, sorting itself into long, smooth, evenly spaced crests along the way.
That sorting is what separates swell from the sea a local blow creates. Wind waves are short, steep and disorganized because they are still being pushed. Swell has outrun its source, so the faces are rounded and the spacing is consistent, and a boat rises and falls with it instead of slamming through it. A large swell running under a light breeze can be a genuinely pleasant day offshore.
Swell rarely arrives alone. Most sea surfaces are a mix of one or more swell trains and whatever the local wind is doing on top of them, and the confused, cross-hatched water that results is far less comfortable than either component would be on its own. Direction matters as much as size: swell on the beam rolls a boat hardest, and a swell running against the current steepens quickly.
Near shore, swell is what turns a workable inlet into a dangerous one. Because it carries energy deep through the water column, it feels the bottom well offshore and stands up over bars and shoals, so breaking waves can appear at an entrance while the open water outside looks calm. Check the swell direction and period against your route, not just the height, before you leave. You can see both on the live marine forecast.
