Although there is some whale watching from Las Galeras and Las Terrenas (mostly from small boats and ONLY when the weather is good with the sea fairly calm), the best whale watching is in Samana Bay with the larger boats leaving from Samana Harbor. However as there are only a few hotels in Samana city itself (and no beaches), most people stay in Las Galeras or even further away in Las Terrenas so they can combine whale watching with beach time.
You can find out more about Whale Watching in Samana and some of the hotels in Las Galeras, Las Terrenas and Samana city at
www.samanaonline.com