That's a difficult question. Since many of the areas affected appear to have been completely wiped out, the 1,000,000 figure is not beyond the range of possibility.
Many areas which were once populated have not even been entered yet. You may remember that, in Florida, some communities were completely shut off from communication for several days. It's much more primitive where this tsunami struck.
We'll never have a truly accurate count, because many bodies will never be found. In addition, records of the inhabitants of many of these regions are incomplete and are now missing, so we may not even know who lived there, or how many.
A million isn't beyond the realm of possibility.
Absolutely right. Those areas were very densely populated.