Hi Steve: I have done a fair bit of whale watching on Canada's west coast.
I recommended the Sigma 150-600 simply because I personally know and use it, but I also respect Ernie's experience and know Tamron make some great lenses in that range too. As Ernie says, any EF or EF-S lens will work fine with your camera. You should have a great reach as that lens range allows you to track whales from a fair distance and reasonably close - from memory, the US law says you have to stay 200m away from them, Canada 100m, but that doesn't stop the whales coming to you - they are intelligent and curious creatures. You don't need an extender or teleconverter. You will need a few spare batteries and cards though...
Much will depend on the weather, and the Sigma is fairly weather resistant and likely so is the Tamron, however your camera is not, so get a wet weather bag to cover the camera if things turn to custard and you are out in a RIB. Even a clean bread bag will do at a pinch.
PREPARATION:
Camera Settings: When you are out there, set your camera to servo autofocus and high-speed continuous - you should have your manual to hand before you go and browse through the information, and look at You Tube videos on settings for photographing wildlife.
Practice with whatever lens you have and if you cannot hold it for a while, do some physical training: generally, unless you are on a very big boat, you can't use a tripod and maybe not even a monopod on a small boat for H&S reasons: certainly not on a RIB (Rigid Inflatable Boat) - see one below to get my point. Try to get on a bigger, cabin cruiser type boat if possible, not just for the room but they often have an upper deck to give you more viewing and shooting options.
I do weights regularly so I can hold my gear steady without fatigue so I don't have to depend on a support.
Study whale behaviour - not only will it help you get better photos, but you will enjoy the experience more. Lots of videos on whales: David Attenborough's videos from his shows are always good.
Some photos I took when I was whale-watching in Victoria, BC:
Whales approach our RIB in hunting mode. What they do is line up and charge their prey (usually seals basking on ice flows) and dive just under the boat, using their bodies to create a big wave that will wash their prey into the sea. They aren't stupid, they know boats behave differently from ice and just roll with it, so they seem to do it for fun or practice. When several did it to our boat they bobbed up on the other side and looked at us as if to say: 'How's that then?" The boats just bob a bit, but you have to be careful not to stand and lose your balance, and either fall in or drop your camera in the drink. Taken at 235mm
Taken at 300mm, this shot shows a pod of whales in almost each step of sounding
There is also other wildlife to be seen, so be ready!
