"I’ve had problems before from ocean nodes being too far inland when there are other waterbodies
and the systems get mixed up a bit in the waterzone’s calculation.
It looks like your ocean is miscalculating with the interplay from all the yellow boxes of the river splines, Its rendering the ocean surface fine when its above the other water bodies, but when thier volumes intersect with the ocean surface it goes funny.
I recon if you stretch your ocean spline out, add a few more nodes and roughly follow inside the coastline of your island,
Might have to play with node positions a little, where the river meets the sea, if you notice the river surface disappear on the last spline section.
Unrelated, the ocean collision extents amount calculates from centre to edge, but the ocean extents are from edge to edge so the collision extent value can be half the ocean extent values.
The collision extents only need to be as big as the playable area, but I don’t know if it really matters on second thought,maybe the equations to check for water collision only uses x y z and size of area doesn’t impact on performance
I’m not sure on the backdrop island issue, maybe the never distance cull setting is overriding the being in the HLOD setting and keeping it out of HLOD so it doesn’t clash,
in game using performance mode it definately culls anything set to not spatially load or set to no distance cull, but HLODs always show."