If they're launching it over the ocean (they would be, if they were launching from Kennedy or Vandenberg) they could just say "YOLO" and let the reactor fall into the ocean. It certainly wouldn't be the first time a nuclear reactor was dumped into the ocean (either by accident or deliberately.) It's obviously not a popular thing to do, but ocean water does provide a lot of shielding..
While true, a reactor blown up over land would also be simpler to clean up; while a reactor scattered over the ocean bed would be easier to leave in place.
"Simpler" here is relative. It would probably still cost billions of dollars and would create enough of a PR disaster to permanently forestall an NTR from ever launching again.
Water is not remotely as good as lead is by mass as a radiation shield. If you aren't already hauling many cubic meters of water for other reasons (and you wouldn't be), it absolutely wouldn't make sense to bring it along solely for that purpose. Water also has the severe problem of being liquid at habitable temperatures, so you either have to continuously spend energy to freeze it or deal with absolutely ruinous slosh.
The linked article is talking about the radiation shielding properties of what essentially amounts to a large swimming pool's worth of water. You know how much that would weigh?? Water is not a good radiation shield, it's just cheap here on Earth, so we use large quantities of it in applications where weight doesn't matter.
