Hey BKK, the only place to watch this highly specialized behavior is on the uppermost tip of Peninsula Valdes in the northern part of Argentina's patagonia. You can fly to Puerto Madryn or Trelew from BsAs and then hire a car or take a bus to Puerto Piramides, the only town on the peninsula itself.
It is tiny and rooms are in high demand - there is a youth hostel with 20-bed rooms and a few expensive hotels. I believe you can also camp near the beach. But, you definitely want to stay in Puerto Piramides instead of farther-away Puerto Madryn and here's why.
It's very rare, even in high season, to witness Orcas hunting for seals this way. To my knowledge the only time of the year it happens is January and February, and a local told me that the best time to go is as close to a late afternoon high tide. Even under those circumstances, you have only a 10% chance of seeing it. So odds are you will need to go for several days to catch it.
It's about a 1.5 hour drive (if I remember correctly) from Piramides to the tip of the peninsula, over unpaved roads that have high potential to mess up your car, which the rental companies will charge you an arm and a leg for. My advice is to make your reservations in Piramides and then once there go about hiring yourself a local who either makes the trip anyway and has room in his car, or find someone willing to drive you to the tip each afternoon. This shouldn't be *too* expensive, especially if you ask around. There is a restaurant owned by an Australian if you follow the one main road away from the town (and away from where you came from Puerto Madryn) who is a lovely person and would no doubt be willing to help you organize such a thing or at least point you in the right direction.
Piramides is a very sweet little town and the peninsula is filled with interesting Patagonian animals, plus in the summer months it's great to be by the beach anyway, so the trip is worth it no matter what. Just be prepared for the disappointment of a no-show, and please tell me if you see them! I too wanted to see it but went in the wrong season.