Put 4 cups water, bay leaves, and salt in a large saucepan and bring to a boil. Add shrimp and boil for 2 minutes. Drain, chop finely, and mix with crab. Set aside.
Prepare Crema de Chipotle. Put the tomatoes, tomatillos, and chiles anchos in a large saucepan and cover with water. Simmer for 5 minutes and drain; reserve cooking water. Pit the garlic and one onion slice in a blender. Add the tomatoes, tomatillos, chiles anchos, chiles chipotles, cumin, salt, and 1 cup of the reserved cooking water. Puree and strain into a large bowl.
Heat the oil in a large saucepan over medium heat. Add the other onion slice; blacken, and discard. Add the sauce to the pan and simmer for 10 minutes, stirring occasionally. Reduce the heat to low and stir in sour cream. Keep warm over low heat.
Preheat oven to 190ºC . Soften the tortillas (see TIP below) and roll tortilla into enchiladas, filling each with about 3 tablespoons shrimp and crab mixture. Bake enchiladas for 8-10 minutes until cooked, but not crisped. Remove and serve three per person, covering each portion with plenty of sauce.
TIP: To soften the tortillas, run them, one at a time, under running water. Shake off excess water, and then place in a hot non-stick frying pan over medium heat. In about 15 seconds, steam will begin to escape; flip the tortilla and heat the over side. Roll immediately.
To soften in the microwave, wrap 5-10 tortillas in plastic bag, and sprinkle with quarter teaspoon of water. Heat for up 45-60 seconds. Roll immediately.