Answer:
I think the answer is -16 feet below.
Explanation:
He was at 11 feet below sea level and then rises 8 feet, which means it should be 11-8 which is 3. And then he descends, (goes under 13 feet) which means 13 is added to 3. 13+3=16 He is under the sea level by 16 feet so it should be -16 feet. Hope this helps.