Dream Imagery
Throughout Meditations on First Philosophy, and especially in the First Meditation, Descartes wonders if he is living in a dream. Accordingly, he goes through mental experiments to figure it out, occasionally describing his environment, more often talking about the imagery and subject matter of dreams reflecting reality. This dream imagery recurs even through the Sixth Meditation.
The Whirlpool
Over the course of the First Meditation, Descartes has mechanically and procedurally dismantled the reliability of reality (in his mind, at least). At the beginning of the Second Meditation, consequently, he says that he feels like he's being swirled around in a dark whirlpool, not sure which way is up. This whirlpool represents the strength and confusion of existential doubt, which he quickly tries to escape by proving the existence of both himself and God.
The River
Descartes uses the image of a river with banks to describe the nature of God's ties with existence itself. Just like Descartes can't imagine a river without banks, so he finds it impossible to even conceive of existence without God. God and existence are necessarily so interlinked and interwoven that it is impossible to separate them; you can't have one without the other. The image of the river also has symbolic meaning: in Scripture, Jesus is called the "living water," and there are many instances of important rivers and river metaphors throughout the Bible.
Wax Imagery
In the Second Meditation, Descartes muses upon the nature of the physical form, using a shapeless glob of wax as an example. He continues using this image for quite a while, examining different aspects of this wax in different contexts, describing various attributes as his argument progresses and evolves. The imagery of wax thus tethers this abstract philosophical work to the tangible world.
