What is the ruling on one who swore by divorce concerning something while his mouth was full of water, then swore by divorce not to seek a fatwa, believing it to be a mere thought?

You are not required to do anything regarding what you mentioned, because the first is doubtful, and the second is a self-thought (waswas), and neither of them holds any weight or on which a ruling can be built. The fundamental principle is the acquittal of responsibility, and certainty is not removed by doubt. It is obligatory to cut off the roots of these obsessive whispers and to turn away from them, and to beware of oaths of divorce, for swearing by them is legally forbidden.