Re: Any manned trip to Mars is going to need a lot of radiation shielding,
"The best radiation shielding is water."
Really? I recall hearing ages ago that Roman era lead was highly sought after as a radiation shield as it was pretty pure and the radioactive isotopes having half lives of around a couple of decades had decayed a great deal in the intervening millennia.*
Of course the advantage is that you can drink water, (although the thieving gets who stole the lead flashing from my block of flats are unlikely to make it into space). And yes, I do realise that as lead is dense getting it into space would take a lot of effort.