Whale watching will require a long lens of the order of up to 300mm for success. If taken from a boat, generally the boat will roll and make it dificult to frame the image. A lens with vibration reduction or image stabilisation would be very useful for this type of photography. A lens with a large aperture will help to keep the shutter speed up to avoid camera movement.
The image above was taken from a boat with a 300mm lens on a very dull day.
The image below was taken from a viewing platform on the shore at the Great Australian Bight where the whales come in close to the shore for breeding every year.