Also: injuries (a lot of people drown by falling unexpectedly into water, hitting a rock or something on the way, suffering an injury that might not be life-threatening on its own but it sure is difficult to swim if you're unconscious or have a broken limb), drunkenness (also a very common contributing factor), or fatigue and hypothermia (a more immediate problem in Norwegian waters than in Greek waters, for example -- cold water kills you a lot faster than warm water).
I wouldn't swim in norwegian waters. What's the point?
Afaik you should avoid swimming soon after a meal or drinking alcohol.