You're handicapped, I flooded hell with a pit from the surface that was two blocks wide. Flooding the entire place is a bad idea anyways, it's a pain in the ass to fight bone serpents and imps while swimming through water because all of it will just sit on top of the lava.
I've flooded Hell using the ocean several times and I've never had any serious trouble. Flipper item + Muramasa and the occasional spamming of the Sun Fury pretty much makes short work of imps and serpents.
I only drained the ocean because it was funny and turning hell into a swimming pool was always one of my short term goals.
If you're going for obsidian just find lava the level right above hell and make yourself an endless stream of water. Or hey just make a obsidian machine using that same exploit.
Proof of concept:
