I've investigated those shadows thinking they were kelps, then clouds. I tried to check everything about water reflection settings or clouds, both in the shaders, sodium and DH. Now I realized they might actually be cave LODs.
If I disable LOD rendering, those big shadows go away. They also only appear past the MC rendering distance.
Graphics mods I'm currently using:
All the required and suggested for Shaders and DH (Sodium, Iris ecc)
DH 2.4.2-b
Shaders: Complementary (Reimagined and Unbound, same results)
Playing in an handmade Cobblemon server (that doesn't have DH).