Swimming
Swimming
[The single panel comic is around 4 times higher than it is wide (317×1284 pixels). A Cueball is watching from the beach while Megan with another character is standing neck deep in water near an extreme drop off (continental shelf?) and another Cueball is swimming further. This part of the comic is at the very top, and the characters are drawn much smaller than usual.]
Megan: It's OK, I can still touch bottom here.
[As the tall image is scrolled down, there are some deep water fish, a jellyfish, and an octopus, a bottom ledge with a beach umbrella on it, and another drop off.]
[Caption below the panel:] I love swimming, but occasionally I realize I don't know how deep the water under me is and it freaks me out.

This comic is about an irrational fear about the depth of water beneath oneself, also known as thalassophobia . Whenever you don't explicitly know how deep the water is, and cannot see the bottom, there is nothing preventing the sea/lake/riverbed from being exceptionally far away. This phenomenon is actually quite common with many bodies of water having a relatively shallow shelf extending a short ways out from land. These typically end with little to no warning, giving rise to the fear that is depicted here.
It is an irrational fear because if one is swimming, the depth of the water underneath is not important to safety as long as one can reliably get back to shore. (This fear may be due to excessively worrying about what happens if one stops swimming, thinking that walking should be safer because almost everyone spends more time walking than swimming, ignoring the fact that the safest thing to do in this case is to keep swimming.) If one is wading, presumably one would feel the bottom drop away. Following the safety saying "Walk out, swim back" would help avoid this situation.
Here, Megan and Cueball are in in the ocean, with Cueball treading water and Megan standing on the seabed, with another girl in the water and another Cueball watching from the beach. Megan mentions that she can still touch bottom, thus thinking it is safe. In front of her however the seabed drops off steeply, becoming nearly vertical. Fish and jellyfish are in the water below, while at the bottom of the frame, but not the sea floor, a small ledge holds an octopus and a beach umbrella.
The beach umbrella may be from the beach, to give human scale. It could also be a Lemmings reference.
The title text alludes to the fact that humans live at or near the bottom of a vast sea of air: the atmosphere.  Every day, most people never rise far from the floor of the atmosphere.  However, this is nowhere near as perilous as descending to the bottom of a sea of water.  Indeed, surviving a rise to the top of the atmosphere requires life support measures.  Also, unlike in water, humans are far too dense to "swim" (fly) in the atmosphere.

[The single panel comic is around 4 times higher than it is wide (317×1284 pixels). A Cueball is watching from the beach while Megan with another character is standing neck deep in water near an extreme drop off (continental shelf?) and another Cueball is swimming further. This part of the comic is at the very top, and the characters are drawn much smaller than usual.]
Megan: It's OK, I can still touch bottom here.
[As the tall image is scrolled down, there are some deep water fish, a jellyfish, and an octopus, a bottom ledge with a beach umbrella on it, and another drop off.]
[Caption below the panel:] I love swimming, but occasionally I realize I don't know how deep the water under me is and it freaks me out.
