That's a great question. To understand the two expressions, it is helpful to look at their origins. The first, "foot in the door", evokes an image of entering an office whose doors are normally closed to you. The second, "wet feet", refers to dipping your toes into a swimming pool before you dive in. Entering the office requires impressing someone, connecting with someone, a stroke of luck, or brilliance on your part. Those obstacles are perhaps too big and you can't do it. However, instead of entering the office, perhaps you can just "get your foot in the door". Here's an example. You want to be a chef in a great restaurant. It is a dream. You have no experience. But, perhaps if you obtain a job there as a mere waiter, you will be able to work your way up to the top. That is what it means to get your foot in the door. This would not be "getting your feet wet" because there are obstacles to attaining your dream. Here's a "foot" example. You want to learn to speak Spanish, but to save time you would rather not take a Spanish course. So you decide to "get your feet wet" by taking a smaller step - listening to some videos for beginners. This would not be "getting your foot in the door" because there are no obstacles. You are learning Spanish, just slowly.