Maybe we first noticed this because we love the phrase "liquefaction of the social body" (1.16.1), but there are a lot of times in the book when water-related words get used to describe people.
Book 1, Chapter 16 provides lots of great examples. Look for the words "stream" and "torrent" in this chapter, such as the huge exodus of people is "a boiling stream of people, a torrent of human beings" (1.16.36). For another example of liquid-related words being used to describe people, at the beginning of Chapter 17, the population of London "poured" out of the city. There were so many people that "the hugest armies Asia has ever seen, would have been but a drop in that current" (1.17.1).
What's with all of the liquid words used in relation to large groups of people? What does it mean?
One thing we noticed is that these large groups of people are only described in liquid terms because they have no order – people stream out because they have no organization or leader to tell them what to do. (Which is what liquid does, right? You don't go to a glass of water and say, "take me to your leader," because water doesn't have a system of government.) Notice how the army isn't described as a stream—only the disorganized refugees are described that way.
Describing people in liquid-y terms also highlights our insignificance. Water is a mass noun. (Quick grammar recap: a mass noun is one that doesn't take the singular form. Water can't be divided; it's grammatically correct to say "Can I have water?" and grammatically incorrect to say "Can I have a water?" Compare that to something like cupcakes, which are individual entities—you say "Can I have a cupcake?" instead of "Can I have cupcake?")
A humans, Wells is suggesting, are individually insignificant when faced with such a massive threat. Humans don't act like ideal, personality filled special snowflakes when the kitty litter hits the fan. They act like animals, or they act like one water droplet in a bucket.