For colorfastness and protection of the leather, you can use a stain and water-resistant spray for leather such as Collonil Carbon Pro Spray, Scotch Guard, Kiwi or Fiebing, should you want to make the leather water-resistant. Test any leather treatment on a small area of the bag and follow instructions carefully to prevent any damage to your piece.
Should your leather bag get wet, the best thing to do is get rid of the water stain before it has time to set. Using a clean, lint-free cloth, soak in as much water as you can by dabbing and not wiping. Allow the bag to dry normally without artificial heat or excessive sunlight. After drying, if the water stain remains, a leather cleaner such as Chamberlain's Straight Cleaner no. 2 can help minimize the water stain. Test any leather treatment on a small area of the bag and follow instructions carefully to prevent any damage to your piece. (Using a leather conditioner may darken the overall appearance of your leather bag, so you may want to clean and condition the entire leather bag based on the severity of the water stain.)
For general care of your tote in keeping the leather replenished, supple, and soft you can condition your bag with a leather conditioner such as Chamberlain's Leather Conditioner Liniment No. 1. This leather conditioner can be used as often as desired to refresh your leather bag.