Black lines are there since SU4 first beta. Actually it’s a very old recurring problem (2021) and it’s been acknowledged by Asobo. I doubt they have a solution.
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The black lines are tiles that not line up with the wave and movement direction and happens sometimes if out of balance and float (i do not know the exact words for it)
The other picture i suppose and guess are current and waterdepth with windshear .
You can try to flatten water in devmode → ground option to see if it makes a change
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Yeah i’m talking about the black lines, but good to know thanks!
Yea. There’s some black lines in the Carribean waterz
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Looks to me like maybe two separate issues: lines versus polygons. My screenshot above is clearly a polygon that changes size but is always a polygon. The others seem to be long lines. Maybe these have the same origin: mismatches of abutting coordinates in some cases two=a line and in other cases three=a polygon. If just one is mismatched, a point is invisible 
Maybe a rounding error or type miscast. Assume calculation of vertices must go from real/floating point to an integer for coordinates.
Took one more image from VR that may have both the polygon and the lines. Lines here maybe look like wind effects. Whatever, it is a bit of a mess.
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