Add all of the marinara sauce and the water. Stir to combine. Simmer the soup, covered, over low to medium heat for 1 hour and 30 minutes to 1 hour and 45 minutes, or until the beets are fork-tender. Your cook time may vary depending on how fresh your beets are and how small you chopped them. (If it’s your first time cooking the soup, you may want to check on it a few times and stir to to make sure it’s not sticking to the bottom of the pot.)