P. 85, "If you fail the roll, you start to drown and suffer one point of damage."
So what happens next? The section on drowning ends here.
Thanks, Tomas.Yes - as it says earlier in the paragraph, you need to roll Endure every turn, and each failed roll means one point of damage. A STR zero, you suffer a non-typical critical injury. However, I would add that die right off if you are kept underwater for another D6 minutes.