Anyone who has seen my previous posts about Drownproofing will be aware that it is possible to survive for hours and even days. But the limiting factor is often hypothermia, as David has pointed out.
Apart from that, there is the the possibility of sunstroke, dehydration, being run down by passing shipping and a relatively small risk of being attacked by sharks or other sea monsters.
In the old days, falling overboard was almost always fatal. Today, the chances aren’t good, but with the advent of GPS, if someone saw you fall in, there is a possibility of rescue and your best bet is to stay close to the scene of the accident. Otherwise it’s just a matter of luck. Big ships depend almost entirely on radar and don’t keep a lookout, so they won’t be any help. If it is your lucky day, you might be found by the local fishing fleet. Those guys spend a lot of time looking at the surface of the ocean.
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