A.T. Robertson Commentary Revelation 16:5

"And I heard the angel of the waters saying, Righteous art thou, who art and who wast, thou Holy One, because thou didst thus judge:" — Revelation 16:5 (ASV)

The angel of the waters (του αγγελου τον υδατων). Genitive case object of ηκουσα. See 7:1 for the four angels in control of the winds and 14:18 for the angel with power over fire. The rabbis spoke also of an angel with power over the earth and another over the sea.

Which art and which wast (ο ων κα ο ην). See this peculiar idiom for God's eternity with ο as relative before ην in 1:4,8; 4:8, but without ο ερχομενος (the coming on, the one who is to be) there for the future as in 11:17.

Thou Holy One (ο οσιος). Nominative form, but vocative case, as often. Note both δικαιος and οσιος applied to God as in 3:1; 15:3f.

Because thou didst thus judge (οτ ταυτα εκρινας). Reason for calling God δικαιος and οσιος. The punishment on the waters is deserved. First aorist active indicative of κρινω, to judge.