Also the 600mm could also double as a paddle in the kayak with a few modifications.

But seriously, the 300mm is going to be much more portable than the 600mm and quite a bit better IQ than your 100-400mm. As far as size I think the 100-400mm would be comparable as bulk goes with the 300mm. Often I carry my 300mm on a black rapid strap and to me it feels very similar to carrying my 70-200mm f/2.8. If you do not have problems with the size of the 100-400 you might be ok with the 300mm f/2.8.

Depending on what Canon does with a 7D II, a 300mm f/2.8 and an imaginary fantasy 7D II might be a very nice portable set up. I have been impressed with the wife's t4i and I wouldn't hesitate a minute taking it and the 300mm on an adventure that requires a light set up and long reach.