Is it permissible to learn diving for contemplation and recreation at a center frequented by foreigners who wear revealing clothing, while attempting to avoid looking and lowering the gaze as much as possible, especially since similar sights, though not to the same degree, are present in daily life?

You are not permitted to go to places where the uncovering of 'awrah (private parts) is prevalent. Your presence there without denouncing the evil is akin to witnessing the evil, especially if you are unable to leave the place. If you give up something for the sake of Allah, He will compensate you with something better. Try to find an alternative diving location that preserves your religious commitment. There is a difference between seeing sights in ordinary, necessary life and going to such places for diving, which is a matter of embellishment.