Just noticed this thread for the first time. I went to South Georgia Island, the Falkland Islands and Chile/Argentina in November 2016 and went through your same dilemma.
This was the solution I came up with:
For the airplane:
I chose the Mindshift Backlight 26L which provides adequate protection, but remains small enough that it could fit beneath a seat if necessary. The riskiest flight was from Chile to the Falklands where they apparently can be very picky about luggage, but I had no problems. The backpack easily fit in a large dry bag for zodiac transport and was on my back every landing. It adds traveling security with rear panel access and can spin on the waist belt allowing lens changes without removing the backpack (nice in muddy or penguin excrement filled fields).
As my "second" carry on, I used the Think Tank "urban disquise 35 classic" which held the K1 and DFA 150-450
I brought three camera bodies:
K3, K3ii and K1
Lenses:
DA 10-17 fisheye
DA 15 for starburst sun shots
FA 31
DA 20-40 limited
DA* 60-250 (modified for full frame)
DFA 150-450
Tripod:
MetFOTO Globetrotter which easily held the 150-450.
Your trip will emphasize icebergs more than my trip, where I suspect a fish eye will really come in handy.
I would absolutely spring for the 150-450 as it honestly shoots very well handheld and is great for pelagic birds following the ship as well as wildlife on shore. I even handheld it shooting from the zodiac without any issues.
On the ship, I shot with the 150-450 paired with the K3ii for albatross, petrels, etc.
On landings, I switched bodies to the K1 for the 150-450 as wildlife isn't afraid of you and will be quite close. I also used the modified 60-250 but used the 150-450 more often (it's sharper).
The 20-40 limited was absolutely my most used lens, hanging from reporter straps off the front of my backpack at all times.
From a seasick standpoint, I wore a scopolamine patch the entire time. We encountered some seas rocky enough that my cabin mate flew out of her bed and a couple of days we weren't allowed on deck. I had no issues and was able to enjoy meals in an empty galley while half the ship was throwing up in their cabins. I find I can stretch and get 4 days out of a given (3 day) patch. If you don't want to look like a dork, you don't really have to put the patch behind your ear (I think that's so ER doctors can quickly figure out what going on if you start acting crazy). I put mine under my clavicle just under my shirt where it's less likely to get rubbed off.
A traveling power strip was also nice, allowing several charges to function at the same time while charging batteries at night.
I wore Gore-Tex chest waders and (non felt bottom) wading boots for landings, hiking miles (we even reproduced the last leg of the Shackleton trek on its 100 year anniversary) with no issues and was never cold.
If I were to go again, the one thing I would absolutely add is the Theta V and it's small tripod/selfie stick!
Links to South Georgia and Falkland albums if interested from a lens choice/outcome standpoint:
South Georgia Island 2016 | Flickr Falkland Islands 2016 | Flickr