To find the position of the penguin relative to sea level, we start with its initial position, which is 21 feet below sea level (represented as -21 feet). When the penguin swims up 9 feet, we add 9 feet to its initial position:
\[ -21 + 9 = -12 \]
Therefore, after swimming up 9 feet, the penguin is now at a position of 12 feet below sea level. Thus, the position of the penguin now relative to sea level is 12 feet below sea level.