water material renders seams and cast shadow option cause water to be opaque

[len_xie]

I'm tring to split part of water surface for fx, but it shows seams in renders, I thought it was caused by shadow, so I turnned off cast shadow, but it turns opaque...
scenes are attached.

[chakarov]

You are right, those are shadows ( if turn sun shadows off, the black line disappear ). If you want to split materials for fx and flat water you'll need to use blend material with a mask (could be an attribute). Splitting surface into separate object could affect reflections, refractions and shadows of water material.

[len_xie]

thank you ! I'm too lazy to do it in comp but you are right should not split them(I know they perfectly match at spliting edge), now I just merge them into one object, and problem gone~
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@chakarov about the "cast shadow " object property, why does it cause the water to be opaque? is it normal?

[chakarov]

Not just merge them. Both meshes should not be split, surface should be one continues mesh.