See
http://www.whalesamana.com/home.html
Kim Beddall, the founder of whale watching in Samana, will give you a real excursion. Her price is at upper end of range, but the tour is worth it. You can get boats to take you for less, but they will just take you to see whales and back. And the least expensive tours are offered in boats that give no protection from sun, wind, rain or waves and, in my opinion, not suited for taking passenges into the outer bay of Samana. What you see from the dock is not what it is like where the whales are.