What is the ruling on uttering the phrase: "O flood, leave my mother and take me"? And is it considered a passing thought not intended as a supplication, making it permissible to say?

It is not permissible to utter the phrase "go, you will not be mourned" to a flood, because it contains an address to it that might imply a belief that it acts by its own will, whereas it is subjugated by Allah's command. Furthermore, it includes supplicating against oneself and wishing for one's own destruction, which the Prophet, peace and blessings be upon him, forbade. The more appropriate course of action is for the servant to ask Allah for well-being for himself and for the Muslims, as the Prophet, peace and blessings be upon him, commanded his uncle Al-Abbas to ask Allah for well-being in this world and the Hereafter.