When creating a warm ocean world or any ocean world for that matter using Floating Islands Buffet settings only in 18w43c and not 43b or 43a (I've tested both and they work fine weirdly enough), it instantly crashes from: 
The game crashed whilst tesselating liquid in world
Error: java.lang.NullPointerException: Tesselating liquid in world
Something important that seems to be worth noting also is this:
10:25:04 net.minecraft.server.MinecraftServer Server thread Preparing spawn area: 9283 | 0 | 0 | 25 (100% 5%)
10:25:11 net.minecraft.server.MinecraftServer Server thread Preparing spawn area: 9283 | 0 | 0 | 81 (100% 18%)
10:25:12 net.minecraft.server.MinecraftServer Server thread Preparing spawn area: 9283 | 0 | 0 | 441 (100% 100%)
In that it automatically assumes it's fully loaded after a certain amount of time despite being nowhere near complete. These aren't all of them but these seemed like the most important ones.