So in answer to these questions about the Eastern ocean they are probably a bit easier to answer since it doesn't require any extrapolation of the text:

• Yes, there is a perpetual wave that arcs upwards from the edge of the world blocking further passage.

• Whilst the mountains are described as "impossibility tall" the wave is not, so you can see over the top to some extent.

I don't believe the Lillys have (or need to have) any deeper symbolic meaning, however I believe their inclusion in the description is intended to achieve a few things:

• They indicate that we have moved away from the rough waves, and the harsh unforgiving sea, to a place where the waters are calm enough for Lilly's to grow.

• They also indicate that the water is freshwater, not salt water.

• The above two points contribute towards a sense of "other worldliness" as these are not things one would normally except in the ocean.

• Lillys are evocative of peace and tranquility, which again helps with the juxtaposition against the previous scenes of danger and risk on the high seas, and also sets the tone for the reveal of Aslan's County.

In terms of a few minor points regarding your question:

• "Stagnant" is probably not the right word, as that usually indicates a pool of water which is not flowing, and has subsequently gone bad - in this case, whilst the wave itself is not moving, I believe the water within it is still flowing.

• "Tidal Wave" is probably not the right word either, since a flat world would not have a tide. Potentially you are thinking of a Tsunami, though that's not technically correct either since the wave was not caused by tectonic activity. I think it would just be enough here to describe it as a regular wave.