"The Bible's claims that Jesus floated up into the clouds or walked on water prove the Bible is a book of myths."
You have already decided (from the accounts of miracles within the bible) that everything in the bible must be ignored as unreliable, fallacious, and mythological in nature. Remember circular reasoning is when the reasoner begins with what they are trying to prove. The thing we are trying to prove is whether God exists or not and miracles have happened or not.
Now to be clear, I am NOT saying, “GOD exists because the bible says so, and we can trust the bible because it is the Word of God.” That is circular reasoning. This is the way I am arguing/reasoning: “Let’s look at some of the other evidence/ claims in the Bible to see how credible the authors are; therefore, we can make an inference to the best explanation of the facts regarding the supernatural claims (i.e. God and miracles) within the Bible."
But notice what you have said, “It's the claims that Jesus floated up into the clouds or walked on water that PROVE the Bible is a book of myths.” You statement here is implying, “We know the bible is false because it says God exists / performs miracles, and we know God doesn’t exist / perform miracles.” Remember whether God exists and performs miracles is the very thing in question and you have asserted the conclusion (i.e. God does not exist and miracles do not happen) from the start. This is a clear case example of circular reasoning.
Perhaps the following will demonstrate how ironic it truly is for atheists who think this way to charge Christians with circular reasoning:
Atheist: "Miracles are not possible because there is no "evidence" for them."
Believer: "Doesn't The New Testament manuscripts record testimony (i.e. "evidence") of many people that witnessed miracles performed by Jesus?
Atheist: "That doesn't count as 'evidence.'"
Believer: "Why not?"
Atheist: "Because miracles are not possible."
Believer: "Do you believe in the 'Big Bang?'"
Believer: "A miracle is described as an effect or extraordinary event in the physical world that surpasses all known human or natural powers. Is it just me or doesn't that sound like an apt description of the Big Bang?"
Atheist: "You're an idiot!"
Do you see the problem with this conversation? The blatant circular reasoning by the atheist--the atheist (reasoned) begins (miracles are not possible because there is no God) with what they are trying to end with (there is no God; therefore miracles are not possible, the bible is not true, and therefore, anybody who believes the bible is “gullible”/an idiot!). All the while, he is completely ignoring or oblivious to the biggest, most obvious miracle of all--THE BIG BANG!