I would suggest using a later snapshot (at least 14w21b I believe), as it has lots more world generation options. It allows you to change almost anything about generation, so you could create a whole overworld that is nether themed.
![World Settings](https://i.stack.imgur.com/l3DFN.jpg)
You don't need to change the map at all. Water World is totally possible to do in survival. I actually play it in HC Survival.
Since villages only spawn their first block layer, rest is cut off due to height limit of 255. So they spawn at y=254. You can get wood from the foundations of the houses.
Also in Swamp biomes you can find witches huts, just the foundation but wood goes all the way down from y=254 to ocean bottom. Lots of wood from just 1 hut!
Also mines spawn inside solid land deep below. Wood can be found in the shafts.
In order to survive water world before 1.13 you have to spawn in an Extreme Hills or Snowy Biome so that the water at 254 is ice. You will spawn on the ice.
There are NO saplings to be found in Water World since village chests cannot spawn.
1.13 -> Even easier, wood is in shipwrecks.
Other tips:
To find seeds, after you get wood make a boat and find an ocean biome. You can tell if you are in one by seeing strange spike tips stick out of the water. It is the only piece of land that you will find in water world. Grass frequently spawns on them.
You can also find sugarcane on the spikes.
Cows can sometimes spawn at the villages, they usually will be floating in the water
Get nametags from mines and go to the nether and find a few chickens from jockeys to perm. Breed them and start a chicken farm.
Also you can start an underwater village, just convert zombie villagers and use glass or make a tube to bring sunlight down from 254 to your base. Obtain lots of stuff from trading with villagers.
Best Answer
Customised worlds were removed in 1.13. The "buffet" type is supposed to replace them in the future, but they don't have the feature you want yet.
Your best option is to generate a world in 1.12, go far out in all directions and then load it in a newer version. That way you won't get kelp, coral, turtles, etc., but many other resources can be obtained in other ways (sweet berries from chest loot, bamboo from fishing and panda spawns, …).