that first links not working, btw. just sends me to spanish-language pinterest
you could include the 'sky' / surface of the water in some of your shots, which would show clearly that youre underwater. you could also have soft beams of light here and there, as there is underwater. you could include some fish, or other sea life, or seaweed - seaweeds particularly useful because of how itll flow upwards, which it clearly wouldnt in air. depending on how long the citys been underwater, you could include barnacles.
you could also make your panel shapes rounder / kind of wobbly, to emulate how water bends light, or even draw with wobblier lines - like how things look when you have water in your eyes. you could make your gutters a dark blue gradient, also.