The note next to the ff says “Glissando on both measures, crossing. I never played La Mer, but my guess is that you need to set the pedals for the notes he indicates, and then play continuous glissandos with both hands going in opposite directions for those two measures. But see if you can contact one or more full time orchestra harpists to see how they play that. What’s the key signature there? I tried to figure out the pedals, but I don’t know the key signature, so I don’t know if G and A are flat, natural, or sharp
I just looked it up on Youtube, and it’s in the key of E major (4 sharps). So my guess is that your pedals, in order to play the harmony that he has written out, would be: E sharp, F natural, G sharp, A flat, B natural, C flat, D natural.