Sherets (H8318) refers to the teeming masses of small creatures that fill the sea and land β the category of living things that swarm in multitudes. Genesis 1 uses it for the creatures filling the waters and the earth, while Leviticus 11 uses it extensively in dietary law discussions about which swarming things are clean or unclean.
Genesis 1:20-21 celebrates: 'Let the waters teem with living creatures [sherets]...' God saw that it was good. The theological significance is that even these smallest, most overlooked creatures are part of God's intentional, ordered creation β made and declared good. Psalm 104:25 marvels at the sea 'teeming with creatures beyond number.' No part of creation is beneath God's notice.