hi there,
Cruise lines do travel south year round but because tropical storms and rain summer months are not the best time to travel.
Between November and May is widely known as the
best time for a trip to South America and destinations south of the equator.
Having said this, if you want to enjoy discounted prices on
South American destinations and you don't mind the weather too much, summer months are the best time (June and September).
Hope this helps
Vacationbuddy