Yes, they might be better moderators of neutron flux, but the point was that they didn’t have it handy at Chernobyl. They had to fly it in, which took time.
In Japan, they have seawater there at their back door in huge quantities.
At Chernobyl, they were flying sand on top of the pile for week, and dropping the sand from a height put more radioactive components into the air.
The Russians were also dumping quantities of lead and boron.
Granted, the first order of business is to keep the fuel from melting into one large critical mass blob. Seawater will work for that but once/if the fuel rods melt, only high boiling point materials (sand, lead, boron) that will dilute and moderate molten uranium have any kind of chance of slowing down the reaction. That is if there is not a critical reaction that becomes explosive or the fuel and contaminated control assemble vaporizes.