I forget which one is where, but there are two pieces to the Wave. The first is in the Planar Quest (where you rescue Haer'Daelis and his troupe of actors from the Planar Prison). The second pieces is in the Underwater City, I believe it's on the King's body if you kill him, but it could be on the Prince.'
As for going back.. Yes you can go back to the Underdark, just go to the "Underdark Exit" on your world map, enter the temple, and keep walking back. I know I went back in after I finished chapter 5 and slaughtered the entire Drow city . Unfortunately, I don't know of any way to go back to the Underwater City, sorry.
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