A closer look
If you manage to enthusiastically slice yourself open while scuba diving, don't panic when you look like an extraterrestrial. Water acts as a massive filter for the light spectrum, and red wavelengths are the absolute weakest, getting entirely absorbed by the time you reach fifty feet down. Without any red light to reflect off your hemoglobin, your bleeding wound will look like it's pumping out bright green ooze. It's just basic physics making your impending shark attack slightly more festive.
