Sorry cant help you in Mar del plata. Why would you look for one there? I can help with most if not all the milongas in BsAs, NY and since you come from Montreal I can tell you that there are "real" milongas there. Where there are traditional tango dancers, many of whom travel between BsAs NY and montreal all the time..
Be aware that you have to check the individual milongas as some are now "nuevo and alternative".
Will you have at least one night in BsAs?

Most of our tango friends in BsAs, travel to Mar Del for the beach and other pursuits as there is life beyond tango...


p.s this being said I will ask around tonight as there must be some milonga there....