Very scary looking holes under my boobs. Does it look like necrosis? (Photo)

I share your concern that this is troublesome and needs to be addressed promptly,  Without an exam it is hard to tell if the hole is in communication with the implant or not. If so, there is a high risk of infection, and using antibiotics makes sense to me. What this clearly is is a "wound dehiscence" or separation. I do not think that necrosis is a significant issue here. What is crucial is if through the open would you can see the the implant.  I am hoping for you that your surgeon is planning to see to this today or tomorrow at the latest.  All the best. 

Based on your photograph it looks as though you may have  exposed breast implant.  If the breast implant is exposed, that is it is visible through the opening in your incision, then revision surgery will be necessary.Of course you must follow-up immediately with your plastic surgeon, this is an emergency.  NextSome surgeons prefer to remove the implant, allow you to heal and place the implant back at a later date.   Other surgeons well excise the  damaged skin, remove the implant cleanse the pocket and attempt to replace the implant.  The second method does carry a greater risk of infection and repeat  extrusion.