Dark squares on top of water next to tall sea grass

I have experienced this as well. I also noticed that the light levels were slightly off. It seems like the seagrass blocks on the water surface have a skylight level of 15 instead of the 14 of a water block. This might be related/causing the the dark spots.

I'm not able to reproduce this in a newly created world. Did you use the world in previous snapshots?

I can definitely reproduce it in a newly created 1.13-pre5 world, specifically the seed -4270258239336733475. It happens in every swamp and is really not to miss in there as it's quite prominent. But if you don't see it, it might be one of the various non-deterministic lighting problems that 1.13 introduced to the set of existing random lighting issues.

Could reproduce. Can be seen best with brightness set to 'moody'.

Could reproduce too. Also appears in blocks near the water; please see.

I don't want to be too optimistic as it could as well be a random error, but it doesn't seem to happen for me in a newly generated 1.13-pre6 world with my seed above.

Cannot reproduce in 1.13-pre6 as well.

Cannot reproduce in MC 1.13-pre6. Seems to be fixed! 🙂