2. As far as the extreme conditions associated with boating is concerned you might want to listen to Martin Bailey's podcast on his trip to Antartica. He recommended a waterproof bag - pricey but if you are shooting with Canon pro gear...
I don't recall any problems carrying cameras in regular Lowepro bags. Waterproof bags are clumsy to open and close, not worth it in Galapagos.
I would go for a 7D with 70-300L, and a 5D with 16-35L at least. Plus a standard such as the 24-105L, and a maybe second lens for the 7D - but there are few that are weather sealed.
If you want BiF then I'd go for either third party TC's (the 70-300 doesn't work with Canon's) or the 100-400 with a Canon 1.4x TC
(not good with a 2x TC in my opinion).
Agreed on the 70-300L. You might make do with one wide-angle, depending on your shooting style.
The 100-400L is not very good even with the 1.4x, and with 7D it won't autofocus with it either. But by itself it works very well with 7D. As for water-sealing, I wouldn't worry about it. It's not that wet there.
Not sure myself if a fast fifty is required - but it's small enough to hardly notice.
I carried one and hardly used it at all. (It was very useful later on the same trip on Equador mainland, though.)
Unless you are really going to concentrate on underwater photography I would take a few rugged waterproof P&S cameras instead - it would give something for the other two people to shoot with if you are only taking two bodies.
I actually missed the "two bodies" part in the original message - probably subconsciously filtered it as impossible. I'd never think of going to Galapagos without at least one body PER PERSON and one spare in case one breaks. If I go there again I'll take two bodies and a waterproof P&S just for myself, and my wife wouldn't be caught dead without at least one DSLR and a P&S of her own.
Seriously: If the kids are even remotely interested in nature photography, buy, borrow or steal at least one more body (even a lowly Rebel). When walking on the islands you really need a camera and a long(ish) lens for every person (200mm is enough for most purposes, BIF excluded). Wide-angle lenses can be shared easier, sceneries wait for you to swap lenses.
And get one bag per SLR, capable of holding it with one long lens, so that everyone (every photographer) carries one. For yourself a little bigger bag with room for the wide-angle lenses &c. Backbacks or good shoulder/sling bags that stay put when jumping off a boat. Forget Pelicans and the like.