Welcome to the forum.
Unless the boat gets very close to the whales, wider angle lenses probably will not be all that useful. For the choices you listed, that 70 - 300 mm gets a lot of good reviews and as you note, can be used with the 1.4x tc. Do you think you may be able to get the 100-400mm or the 150-600mm in the near future (assuming good weather and bright skies)?
Another option is to rent or borrow another body, put a wider angle lens on it and keep it close by while you use the longer lens. Then, if you want some landscape style shots or the whales do get close, you have it ready to go.
