There is not a single reason asking for verbal confirmation that the water is water would make sense. If Zerxus had ill intentions, he'd lie and claim it was. If it was just water, all asking would do would establish Steve as actively mistrusting him.

Besides, it's Steve.

He makes a low, indecisive noise in response to the question that echos in the goblet. Sets it back down on the table when he's drained it. "You know, you're the first person to ask that." He's touched - however weird that is. "I never thawed out enough to fully wake up, but it wasn't a controlled environment or static temperature, so I was up and down. Why?"