Since Noah preached many years while building the ark, one assumes folks had some general idea, but did not believe it. Possibly one of the biggest structures in the pre flood world, the ark would likely have been seen for miles in several directions. I don't recall that Noah got some message to flee to the wilderness to build the ark either.
If there were slow rains, the wicked would have had weeks to try to burn or break into or somehow destroy the ark. They had metal in those days, so they had metal implements to do so. The pitch on the ark was likely some sort of accelerant to fire, and attacking a wooden structure with pitch inside and out in a mere slow rain would probably be no great deterrence.
I would say the picture is one od sudden destruction, where oceans of water poured down, and gushed up, utterly destroying life in short order...even though some folks would have sought higher ground in a hurry, and possibly survived some days longer.