Once again, no direct experience on my part, but I would imagine that neither of your lenses would be anything long enough - I doubt if the boat "driver" would want to get that close to such large creatures.
As you have 20 MP to play with, you could crop in a bit, but a more powerful lens would be a better option.
If you want to stick with Canon, the most flexible option would be their 100 - 400 "dustsucker", so called as it is an ancient design and has a trombone style zooming action which tends to pull in dust. It is compatible with a Canon extender to increase its reach, but its relatively small aperture would probably mean that you would lose the AF function. Despite its age, the lens is renowned for its sharpness. It is quite heavy, so you will possibly want to use it with some form of support, a monopod for example. A tripod might not be practical on a boat unless it's an exclusive hire.
http://www.amazon.co.uk/Canon-100-400mm ... on+100+400The new 200 - 400 is a stellar performer and has a built in converter which will work with the AF, but is an eye watering price. Probably one to hire if you are interested.
http://www.dalephotographic.co.uk/mall/ ... 5AA/142365With a bit less reach at the long end, the 70 - 300 is also a noted performer and is compatible with Canon's extenders, but once again there are potential AF issues.
http://www.amazon.co.uk/Canon-70-300-4- ... 0+400mm+f4There is a consumer lens option of a 70 - 300, which receives good reports, although I have no personal knowledge of it.
http://www.amazon.co.uk/Canon-70-300mm- ... 300+is+usmThe Sigma 120 - 400 also gets good reports and is considerably cheaper than the Canon equivalent. I have seen it recommended and it is probably the best third party lens, although I am always cautious when it comes to Sigma lenses. There is a 150 - 500 model, but I found the AF to be slow when I tried it and it is not an especially good performer at the long end, so there might be little advantage over something shorter and cropping in a bit.
http://www.amazon.co.uk/Sigma-120-400mm ... ma+120+400I have recommended zooms for their flexibility. Canon does make long prime lenses, but weight and size could be an issue, not only for carrying around and using when you arrive. Anything checked into the hold on the plane runs the risk of not turning up at the other end, but a large might fall foul of hand luggage restrictions.
Whatever you end up doing, it is worthwhile getting a filter to protect the front element of the lens from the sea spray. The filter will be easier to clean and can be chucked if really necessary. If you hire a lens, make sure that you are insured and that the cover applies to use on a boat.