Comment by skykooler

One possibility is the use of a cycler - in other words, putting only a couple Starships with massive amounts of water onboard in an orbit that loops back and forth between the Earth and Mars. Passengers would board it for the transit, then transfer back into an unshielded starship that flies in formation for the journey. This way, the shielding mass only needs to be launched once, and can be reused for an arbitrary number of flights.