In a large skillet, brown the ground beef and onion over medium heat until the meat is no longer pink. Drain. Add the jalapeños, water and taco seasoning, increase the heat to medium-high and cook, stirring frequently, until the liquid has evaporated. Remove from heat.
In a 4 to 6-quart slow cooker, add the prepared beef mixture, the Velveeta cheese and the cans of Rotel. Stir well and cook on low until the cheese is completely melted, 1 to 2 hours. Keep the slow cooker set to warm to serve during a party, giving the mixture a good stir before serving.