by Marcuse » Sat Jun 21, 2014 5:14 pm
Well Lovecraft had a very particular style and modus operandi. Anything that was unspeakable, unknowable without causing insanity, or outside the range of human knowledge and capacity to understand was very common.
Creatures like Chthulu, a winged octopus quasi-deity that was said to dwell in the sunken city of Rl'yeh kind of fits the "underwater alien" thing, given that it's pretty much stated outright in The Mountains of Madness that his race came from space. The Old Ones related in detail in that story also came from space, and ended up living underwater.
Several of his other stories were things like reanimating the dead, long lost reptilian races preparing to sweep away the faltering civilisations of man, stuff like that.
Basically, anything that paints humanity as a small, weak, and powerless species in a world where elder races trod the Earth far before we did, and we're living on borrowed time before they take over. In a sense, they're not "alien invasion" because the aliens are said to have landed sometimes millions of years before we evolved on Earth, so it's less an invasion, more the rightful owners of the planet sweeping away the vermin that have infested their former lands.
I suppose a comparison might be useful.
An alien invasion would be a hostile species of alien arriving at Earth and attacking. Like Independence Day.
A Lovecraftian horror around the same thing would more likely be a lone scientist or soldier stumbling almost by accident on some hitherto unknown subterranean archive of records of a species of aliens that used to exist on Earth, and their dwindling remnants have been hibernating ever since their fall milennia before humans even evolved. He almost wakes them up, but attracts the attention of something hideous and sanity draining, that he is reluctant to put into words. The impression is left that, if awoken, we'd all be doomed.