Load up on non-starchy veggies
Starchy veggies (like potatoes) and processed whole grains (like whole-wheat bread) are foods I'd normally recommend eating in moderation, since they provide plenty of nutrients, fiber, and healthy carbs. However, high-carb foods aren't your best friend when you're looking to drop water weight. Essentially, when your body stores excess carbs, it stores them with water. So replacing carb-heavy foods with non-starchy veggies that still provide filling fiber without as much water retention is the way to go. For a week before your event, you can swap out the starchy carbs for more non-starchy vegetables to lose some water weight.
Aim for filling at least half of your plate with non-starchy veggies like asparagus, peppers, broccoli, cauliflower, mushrooms, spinach, kale, cucumbers, and more. This is a great idea for everyone's health, not just people wanting to lose weight.