Cool. So the spaceship would be more of a small planet?
I mean, if they feel it would require 6 feet of lead, that would be 72 feet of water by your ratio.
22 metres of water. Assuming a spherical spaceship, with a living space of, oh, 20 metres in diametre (yes, just a WAG, I looked around, I couldn't find any estimates for transit vehicle sizes in various proposals like Mars One), that would be:
4/3*pi*42^3 - 4/3*pi*20^3 or 277k cubic metres of water, therefore 277k tonnes of water.
So, Wikipedia helpfully offers this calculation:
https://en.wikipedia.org/wiki/Spacecraft_propulsion#Power_to_thrust_ratio
Of 620kg of fuel mass to push a 10 tonne spacecraft. That presumably means a 277k tonne spacecraft would require 17 tonnes of fuel, not including the mass of the living quarters, nuclear power plant, thrusters, whatever... Maybe 20 tonnes of fuel?
That doesn't sound that horrible actually.
And we get a pretty pool too! Maybe it'd even sparkle with the lights off from high energy particles crashing into it. Just don't dive too far down, or you'd get irradiated as well as run out of air? :)