Update:
With the introduction of the ocean biome in beta 1.8, it is possible to play in a "water world" without needing to raise the sea level, by finding a seed which starts you in the middle of such a biome.
Seeds can be found wherever Minecraftians congregate, including the official forums and Reddit .
Even with a world editor, you cannot flood "the whole world", because the world is procedurally generated as you explore it.
The best you could get would be to flood the areas of the world you've already seen.
Instead, you should look for mods which affect world generation.
A quick search of the mod forum turns up the Phoenix Terrain Mod , which has a waterLevel setting (default 64) as well as many others which may help you achieve the effect you're looking for.
gaming.stackexchange.com/users/1980/Ben Blank
Try out the Bukkit plugin called Terrain Control .
It features the ability to change the water height by modifying the config.
However, you will have to run a Bukkit server on your local network to use it.
As of snapshot 14w17a , it is now possible to customize the water level ingame.
When creating a new world, this can be achieved by clicking More World Options... , changing the World Type to Customized , and then selecting the Customize button.
Here, you can change the sea level by moving the slider that says Sea Level , as well as many more options.
gaming.stackexchange.com/users/46453/tomb
The only possible way i can think of that might work for this is if you use the WorldEdit Mod, The only issue being though that this won't raise the properties for the actual world as i'm sure you've guessed.
It'd be a very quick and easy way to modify the water levels in the immediate and surrounding areas though.
http://wiki.sk89q.com/wiki/WorldEdit
I hope you've got a few days spare
, It's a hell of a mod to learn!
gaming.stackexchange.com/users/46432/HWGMousey
You can create a water world by going into the superflat customization screen ( Create World > More world option...
>
World type:
Superflat > Customize ) and then choosing the "Water World" preset.
Note that this will generate villages, so you might be able to get some wood and stuff by doing some diving.
Have fun!
gaming.stackexchange.com/users/41882/Nathan2055
There is a naturally generated survival island world that I play on occasionally.
The seed is "Sprite".
the S is capitalized.
gaming.stackexchange.com/users/44214/jacen.garriss
Just create a customized world, and on the FIRST PAGE, and the FIRST SETTING, there will be a slider bar that lets you change the sea level.
I seem to be the only one that can figure this out.
gaming.stackexchange.com/users/104608/911Riley1
I believe in a snapshot before 1.8 there was a cusomize world option where you could just make a world to your specifications
gaming.stackexchange.com/users/104611/DaanRio
I would like to play in a water world with deep oceans and islands everywhere, not just occasional ocean biomes. I have looked for a good seed, but nothing really shows up. I am wondering if there is a way I can hack to flood an entire world. Is there an effective way of raising the water level across the entire world?
What is an effective way to raise the water level on an entire world?
gaming.stackexchange.com
<minecraft-java-edition>
gaming.stackexchange.com/questions/26513
gaming.stackexchange.com/users/10210/Wulfhart
There is a Bukkit plugin called Terrain Control whic has the ability to change water height. Create a customized world and there will be a sider bar that lets you change the sea level. Changing the water level is possible through Terrain Mods or costumization of the world.
Available features in Mods enable  changing the water level.