The serpents only spawn in Ocean biomes, and only if it's nighttime or/and stormy. So, it's a good idea to keep an eye on the minimap if sailing under either of the last two conditions -- unless you're specifically hunting serpents, of course.

One literally ate me last night.
I fell off our boat during a storm, because we were fighting the thing with bows so I wasn't sitting or holding fast, thanks to either lag or a wave (it happened too fast for me to know which). Boat was at full speed so no chance of climbing back on board.
Just before I drowned the serpent closed its jaws around me.
Thankfully, my friends made it to shore and managed to build a portal to the base.