Travelling to the Galapagos Islands in a few short weeks with the wife and two teenagers, all de facto photo enthusiasts
I was there... what, 12 years ago. For photo/wildlife enthusiast, it's wonderful.
I had EOS 3 back then, 100-400L, 28-135IS and 50/1.4 if memory serves,
my wife carried EOS 300 and 75-300IS (and some short lens I've forgotten).
We will each have our own camera body, and obviously sharing lenses
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for the bodies, and 70-200, 100-400, 16-35 and 24-70 zooms I propose, laptop, external hard drives. & accessories we plan on sharing
That doesn't sound much for four people. I would expect that to lead to serious
fights about the long lenses, especially the 100-400, if there's more than one
person with any interest in birds. To save weight I would leave the laptop and HDDs though,
just take a lot of memory cards (I mean a *LOT* - I'd plan on at least 1000 pictures per person per day).
And plenty of spare batteries, too. (You may not be able to charge them on the boat - check in advance.)
Also, any suggestions/personal experiences on an underwater camera/housing for a small point and shoot possibly? We will have opportunities for snorkeling (sharks / seals are abundant photo ops) and would add a P/S if people like one enough to convince me.
Absolutely! We didn't have any underwater photo gear then and have regretted it ever since. :-)
I'd suggest forgetting underwater housings and getting something like Canon D20 (I've got a D10, which is fine, the D20 is smaller).
I am still debating the value (weight) of bringing my Induro GHB2 head (3+ lbs), Manfrotto 055 Carbon Fiber - Q90 - 4 Section tripod (another 3+ lbs) and /or monopod - versus lighter options.
As I recall, I had hardly any opportunities to use a tripod. If I was going again, I'd leave it at home.
A monopod would be more useful but not really essential either. For most of the time there's plenty
of light - I had no trouble shooting ISO 100 slide film handheld almost all the time.
I am also CPS member who could 'evaluate' just about any lens for a week, but with trip overlapping with Olympics, selection/availability might not be great. Was thinking about 28-300, so if Neuro (or others) have experience with that, let me know. I am considering a trial of this lens to help limit my weight, but I love my 70-200 and 100-400, AND if I bring 3 white whales with me, not sure my wife will still want to be seen with me 
I'm sure my wife would never forgive me if we went there again without at least one long white lens for each of us. :-)
If your kids are really into photography, you can really expect fights over the long lenses.
If at all possible, get at least one more tele lens. (I'd get two, so that there'd be one for
every person, but perhaps your wife isn't as much into long lenses as mine).
The 28-300 (never tried it) would probably be great, but as a lighter alternative, consider the 70-300L IS. If weight becomes limiting, drop the 16-35.
In any case, good luck, and have fun! There aren't many better places in the world for
wildlife photography.