And I looked,
The previous verse, John "heard". Now he takes a look, probably because the vision contains a Person he is familiar with.
and BEHOLD, a WHITE CLOUD,
BEHOLD means PAY ATTENTION. The cloud indicates the Shekinah or "glory of God". Recall that the waters above the firmanent probably are clouds (Genesis 1:7).
WHITE denotes PURITY.
and sitting on the CLOUD was ONE like a SON OF MAN,
When One is reigning in judgment, the leader, king, or judge usually is seated. Here the Son of Man is SITTING waiting for the pending judgment/harvest. This Son of Man described this scene in Matthew 24:29-31.
There are two contrasts here. First one of water, and second of the Son of Man.
Water:
The Son of Man sits on the WHITE CLOUD; in Revelation 17:1,15, the harlot sits on many waters. If water is related to spirits' abode, then the cloud denotes the presence of the Holy Spirit.
Son of Man:
Compare the use of "Son of Man" by Christ in Matthew 13:36-37-43.
On this earth in poverty (Matthew 8:20)
Made to wear a "crown of thorns" (Matthew 27:29)
Sitting on white cloud with POWER (sickle)
Sitting here with a GOLDEN CROWN.
having a GOLDEN CROWN on HIS HEAD, and
See Psalm 21:3 for the prophetic version of this KINGLY picture. Wearing the CROWN denotes ultimate authority to carry out the task at hand.
a sharp SICKLE in HIS HAND.
This description replicates that given in Joel 3:9-13-16.
One general observation: In the Old Testament, the feast of the tabernacles followed the harvest. So here, too, the harvest comes before the celebration (Revelation 19:1-10).