Hi Christoph1, I don't know where are you going. South America is really huge. In Argentina, there is a "diving hub" as you defined. It's Puerto Madryn, located something like 1800 km from Buenos Aires.
There there are several diving spots where you can dive. Several wrecks like "Albatros", "Miralles" and "Folias". You can dive with sea lions (both diving and snorkeling), and, depending on the part of the year you will be visiting, you can also make whale watching. Only at the end of the summer you could dive wet, otherwise, and depending of the amount of days you will be diving, you should dive dry. From there, another place near is Puerto Piramides (150 km from Puerto Madryn), waters have better visibility normally. There are operators located in puerto Piramides, but there are also operators from Puerto Madryn that carry you to Piramides.
Besides Argentina, Brasil has many diving hubs, Chile, Colombia, Peru, etc., but I leave those descriptions to the locals.