Question

What is the difference between a sea and a swell?

Answer 1

It is as written in the two answers above; a sea is a large body of water. But, although English is not my mother tongue, I have sailed for half a century and I know that sometimes a wave is called a sea.

If that is what you were thinking of then the difference between a sea (a wave) and a swell is in its amplitude. A swell is the result of a strong and persistent wind happening far away. By the times it reaches you, it is a train of low amplitude but fast moving waves. The speed at which waves travel in knots is 1.34 times the square root of the distance between their peeks in feet. This is also what limits the speed of a displacement boat.

So you may have at one place, a swell coming from one direction and waves (or seas) coming from another one. I remember once crossing the bay of Biscay with my sailboat, into a northerly wind but a westerly swell. There was a small coastal cargo ship working his way to the west, against the swell, with water breaking over the entire ship. At time the entire bow was buried under green water. But for me, a much smaller craft, the swell had no other effect than hiding at time the horizon. I went gently up and down.