That’s a great explanation of it’s an articulation disorder, which it seems to not be, as OP says “water” and “what” fine later on. It’s a very classic stutter block due to being a Covert stutterer, which is when the idea of others hearing your repetitions is so unpleasant, you subconsciously start avoiding and switching words, and/or fight to get the word out. But certain words are unavoidable. I bet OP also struggles to say their own name, and that a proper noun of something that’s important to OP starts with a similar sound.