Since you're asking this question, I'm going to assume you haven't done this type of photography before. Given that fact, your T6i and the lenses you mention won't be the limiting factor on the shots you get. I'm no expert but I've been a few times with equipment comparable to yours and have achieved pictures I'm happy with.

The challenges I found were predicting where the animals would surface, and maintaining a stable base to shoot from on a pitching boat. If you've done this kind of photography less than a few times, I reckon those will be your limiting factors.

My tips for whatever they are worth

1. Set your camera to AF, centre point only. Area AF will be confused by the sea and unless you're very fast at MF they'll be gone before you find your target.
2. The bright surface will tend to confuse your metering, expect to apply some positive compensation, I'd expect a stop or so although best to take some test shots. Consider manual exposure if lighting is consistent.
3. Stand with your feet shoulder width apart and try not to fight the motion of the boat, you'll get less shake by going with it.
4. Where on dry land you would want roughly 1/focal length to freeze motion, remember that you have to freeze the motion of the boat too. I'm not sure what modes your canon has available, on my Pentax I'd set TAv 1/1000 f8 and let the camera handle the ISO, opening the aperture up a bit if I had to to keep the ISO down to a sensible level

Unless you're very lucky, the 75-300 is probably your best bet. I have been lucky enough to want 18mm on a dolphin boat, but only once. Good luck.