I made this using tnt to terraform a massive cave in a superflat world (/gamerule DoTiledrops False is very useful), then added decoration, so gravel, kelp etc.
-Probably works in all snapshot versions (1.13)...

-Does use kelp, seagrass, turtles, and new borderless slab modelling underwater (stuff not in 1.12)
-Doesn't use the new prismarine slabs etc.

SuperFlat layers:

1 Grass
2 Dirt
56 Stone
1 Bedrock

A few problems include how leaves (being transparent blocks and all that) don't interact with water correctly, creating the water edge texture, when it is next to air.