There isn't anything inherently demanding about the water levels in Sonic. They are as fun as the most average water level - which is to say, annoyingly frustrating - and are relatively simple.
However, the stress is amplified immensely by one thing: The music, and a very specific sound effect.
As Sonic begins to drown, a counter appears on the screen alongside some of the most anxiety-inducing music ever put in a video game. It appears out of nowhere and incites pure panic.
Give it a listen even without the context of playing, and try to tell me your blood pressure hasn't skyrocketed.