Well it is a definite texture bug, it looks like there is a transparent 2d texture of the sea grass instead of the water texture it should have, I believe this can only happen when the world loads and cannot be done by hand. It only happens when a sea grass is at the top on the surface. The water works normally and functions fine, Placing blocks, placing water, sponge, or breaking sea grass will fix the water.

adding that placing coral or kelp, will change what the texture shows. If the plant is placed in the same place