Pretty sure its a night only enemy, and maybe possible if your inside a storm, otherwise its quite rng. Have sailed a ENTIRE night, also part inside a storm and no serpent, yet 1 other time I didn't want to deal with serpents and I outsailed past 1, sailed past a 2ND!!!, and a 3rd spawned and decided "f it" and led the thing to shore for some harpooning...finally got serpent shield at least.