Answer:
Mis amigos y yo estábamos nadando en la playa cuando vimos el tiburón.
Explanation:
Mis amigos y yo estábamos nadando en la playa cuando vimos el tiburón. This means in English: My friends and I were swimming at the beach when we saw the shark. The imperfect here is used to talk about an action in the past which was interrupted by some other action. In this case, the swimming was interrupted by the appearance of the shark. So the first verbs goes in the imperfect and the second, the one that interrupts, in the preterite.
