Spawn conditions are ocean biome; rainstorm at night or thunderstorm during the day. Day spawned serpents won't despawn, according to the wiki night serpents will despawn during the day.

Storms seam to be more common in the ocean, but you cannot count on a thunderstorm every time. Take advantage when you have a storm and get out on the water. I find half sail to be the best speed, plus view of the horizon is not blocked. You still have to keep an eye out and be ready to stop.