The ISS is situated safely deep inside the Earth's magnetic sheath which protects you from a lot of high energy radiation though. You can use mass for shielding

The ISS is situated safely deep inside the Earth's magnetic sheath which protects you from a lot of high energy radiation though. You can use mass for shielding but you also desperately need that mass for cargo too and most cargo isn't going to be as dense as water which makes good shielding. The ISS has some fancier lightweight shielding but has issues with secondary radiation from particles hitting the metal skin of the modules which we'd expect to see on Starship too but higher since it'd be in interplanetary space instead of nicely close to Earth like the ISS.