Originally posted by Veseljko:

Hmm. I have found a lot of serpents on a sunny day in completely still waters. If what you say is true, those are the ones that appeared in storms and stayed at the location for days until i come their way again.

You or your friend probably sailed there before during the storm and he was spawned, All mobs that spawned during the night disappear during the day if you go outside their drawing distance.
but if he was spawned in storm he dont despawn. Serpent may appear at the drawing boundary, you may not even be aware that he has appeared.

another example
**wolf may appear only at night, you cant find **durning a day.