Is it possible to make a Minecraft world without land?

I want to make a custom Minecraft world based on the movie WaterWorld. I want to first make some ruins then place them in the bottom of the ocean. Is it possible to make it so that the will spawn randomly underwater and have there be no above water features? Am I making sense?

I don't believe you can without a mod or knowing a specific seed where you spawn in the water (they're rare but I've seen it happen). If you set it to large biomes and go find an ocean one you could probably do the same thing though.

You can create an ocean world with the superflat world generator. It will basically be an infinite ocean with no land at all. You could also try the large biomes option like the other person suggested.