A Vampiry Shipwreck

Riddle: You are floating in cold freezing water in the ocean after a shipwreck. Suddenly, out of the blue, you see an island. You quickly swim towards the island and sleep on the cold sand. The next day you see a boy, a woman, and a man who had claimed they had been poisoned and they need your water to cure them you ask them "What poison can be killed by water?" Then, you feel thirsty take out your water bottle and before you drink you realize it is some sort of potion then understand why they went to you. You see a note which tells that two are vampires while one is an actual human and tells you a secret code that could be a hint. The code was manacure. Who should you save?