heavy
gentle, slight
10-foot, 20-foot, etc.
Atlantic, ocean
The ship was bobbing up and down like a small boat in a heavy ocean swell.

break, come in, roll in, surge up
The Atlantic swell surged up under them.

in a/the swell
The trawler rolled wildly in the heavy swell.

on the swell
with the swell
The boat rose and fell with the swell.

gentle, soft

swell of
the gentle swell of her breasts