Brown the sausage: Heat a large deep skillet or Dutch oven over medium-high. Add the Italian sausage and cook, breaking it up, until browned and cooked through. Drain off excess fat if there is more than a tablespoon or so.
Build the sauce: Pour in the marinara sauce and water. Scrape up the browned bits, then bring to a strong simmer.
Cook the tortellini: Stir in the cheese tortellini, making sure it is submerged. Cover and simmer, stirring once halfway, until the pasta is tender and the liquid has reduced.
Make it creamy: Kill the heat. Scatter the cubed, softened cream cheese over the top, let it sit a minute to soften, then stir until fully melted and the sauce turns glossy and pink. Let it rest a few minutes to thicken before serving.