The answer would be “nadaba” because you use the imperfect form. The imperfect is used when something happens for more than an instant in time. It is also used for reoccurring things.
The correct answer is, Todos los veranos Juan nada en el mar.
Explanation:
This is because you aren't talking about yourself or anyone else in the past. Juan is swimming in the present, for example, you were there at the beach with a friend and you tell your friend what Juan is doing. You would say Juan is swimming in the ocean.
