Using sea water as additional coolant source isn't a new idea.
That's always been part of US Submarine nuclear reactor safety steps that could be taken in the direst situation. "Disposable, sinkable container" if necessary. (Hasn't been necessary)

You'd really rather not use seawater unless all the distilled water sources are gone. Injecting sea water into a formerly corrosion-controlled system will render the system unusable in future. (chloride cracking corrosion will occur and the piping can never be trusted for ops ever again)

The reason to use seawater, or any water, is to keep the core covered in order to dissipate decay heat so that the fuel remains in its original geometry near the control rods and remains sub-critical.

Molten fuel can puddle and and go critical/super critical and once again produce heat beyond control. (The so-called "china syndrome" where the hot fuel "melts its way to China".)