Hi Lorraine,
I’m happy to answer your question. The best time to view and swim with the whales is when they are on the Silver Bank, between January and April every year. During that time thousands of whales visit the Sanctuary for the Marine Mammals of the Dominican Republic, during the week our guests are out there with us we see a few hundred.
You can see the exact dates here: http://consciousbreathadventures.com/schedule-availability/
Thanks for posting!