Place ground turkey in a large saute pan over medium high heat, breaking up with a spoon until cooked through. Season with Italian seasoning. Add marinara sauce and stir until combined. Remove from heat.
Ladle some sauce onto the bottom of the slow cooker. Add a layer of noodles. You will need to break some to get more coverage. Next add dollops of ricotta and a sprinkle of mozzarella cheese.
Repeat this until you have at least 3 layers. Pour the remaining sauce on top and sprinkle heavily with mozzarella cheese. Pour water around the edges of the lasagna.
Cook on high heat for 4 hours (or low heat for 8 hours). Let lasagna cool 15-20 minutes before serving. The longer you let it cool, the more it will firm up and be easier to cut into pieces.