16:5And I heard the angel of the waters saying, "Righteous art Thou, who art and who wast, O Holy One, because Thou didst judge these things;

And I heard the ANGEL of the WATERS saying,

It is not unusual for angels to be in charge of physical phenomena:

Revelation 7:1
Revelation 14:18
Revelation 16:5

However, if a person agrees that water is the "container of spirits", then this one is especially significant. Particularly as a witness during this last half of the tribulation, this is a testimony to the spirit, blood, and water (1 John 5:6-12).

This angel probably was the one who purified the waters at Jericho at the word of Elisha (2 Kings 2:19-22).

"RIGHTEOUS art THOU, who ART and who WAST, O HOLY ONE, because Thou didst JUDGE these things;

This angel affirms how RIGHTEOUS Jesus Christ is. Prior to Jesus' blood being shed on Calvary, God exhibited this same RIGHTEOUSNESS as we find in the discussion of Sodom (Genesis 18:22-25-33). Son like Father in this instance. Clearly, from John 5:17-22-24, the HOLY ONE, who is the JUDGE of these things is Jesus Himself.

Watchman Nee points out that the phrase "who is to come" (Revelation 1:8 and Revelation 4:8) is missing here (as well as in Revelation 11:17), so he concludes the return-of-Christ event takes place sometime between chapter 4 and the 7th trumpet. "Who is to come" has already come.