Toast the curry powder, sea salt, coriander, ginger, and turmeric in a medium stockpot over medium heat for 30 seconds, stirring constantly.
Add the yellow onion and 1/3 cup of the water, and cook for 5 minutes, or until the onion is soft and translucent, stirring frequently.
Whisk in the coconut milk and the remaining 2/3 cup water, and bring to a boil.
Add the sweet potatoes, decrease the heat to medium-low, cover, and rapidly simmer for 10-15 minutes, or until the sweet potatoes are fork-tender, stirring occasionally.
Stir in the scallions, raisins (if using), and cilantro, and continue to simmer, uncovered, for 5 minutes, or until the sweet potatoes begin to break down into the sauce, stirring frequently. Remove from the heat.
Spoon the curry into bowls and serve with warm brown rice and lime wedges, if desired.