> That water was drained, precisely because of concerns that it would melt down and come into contact with water.
Even if the concrete base under the reactor was breached, the water underneath had been drained. In retrospect this was unnecessary, but it added another layer of precaution.
Furthermore, what makes you think a potential explosion after coming into contact with water would be "massively" worse than the original reactor explosion? In case you aren't aware, the RBMK-1000 was a water cooled reactor. The explosion happened because the temperatures inside became so high that the pressure vessel ruptured. By comparison, the contact with the water beneath the reactor hall would not be trapped in a pressure vessel and wouldn't result generate such pressure.
What actually produced the most amount of nuclear contamination was the period of time when the fuel rods were exposed to atmosphere and burned. This put large amounts of contaminants - radioactive smoke, essentially - into the atmopshere.