Well, just to make sure you know, serpent stew is not the best food in the game (Health food, I assume you mean) that honor belongs to Misthare Supreme. Under that is a tie for 2nd between Honey-glazed Chicken and Meat Platter, and in a close 3rd (Providing the same HP/STAM as the Chicken/Meat platter, but lower HP regen) is Serpent stew.

The Serpent stew's big advantage is how early it's available.

If you want to find serpents, just sail, that's all there really is to it. They're most common at night during a storm, but some do spawn during the day, only once however, when that serpent is killed it will no longer spawn (but night/storm serpents still will)