en_tn/isa/48/18.md

If only you had obeyed my commandments

Yahweh describes something that could have happened but did not. (See: rc://en/ta/man/jit/figs-hypo)

Then your peace and prosperity would have flowed like a river, and your righteousness like the waves of the sea

These two phrases share similar meanings. In both, Yahweh speaks of Israel's experiencing abundant blessings as if those blessings flowed like water. (See: rc://en/ta/man/jit/figs-metaphor and rc://en/ta/man/jit/figs-parallelism)

your righteousness like the waves of the sea

The verb may be supplied from the previous phrase. Alternate translation: "your righteousness would have flowed like the waves of the sea" (See: rc://en/ta/man/jit/figs-ellipsis)

righteousness

This is probably a metonym for the salvation that come from righteousness. Alternate translation: "salvation" (See: rc://en/ta/man/jit/figs-metonymy)