Psalms 71:20

Quicken me. — According to the written text, quicken us, an indication that the psalm is a hymn for congregational use. As for the change from singular to plural, that is common enough.

Depths... — Abysses, properly of water. (See Psalms 33:7.) Perhaps here with thought of the waters on which the earth was supposed to rest. If so, the image is the common one of a “sea of trouble.”