love the 24-105, when I only bring one lens with me on a climb, this is the one. The 100-400 is definitely a compromise - not as sharp as the 400 f5.6 prime, more expensive and heavier than the 70-300. But for me it's a very good compromise. You get more reach than the 70-300 plus it's quite a bit sharper. And compared to the 400 prime you have the zoom (Id estimate that half the shots I've taken with it are at 400, the other half at something else), Personally, I recommend it.
For whale watching from a boat I've gotten my best shots with the 24-105 (well at least if you're fairly close), be sure not to leave it at home.