I went in 2014 on a 15 day with Galapagos Travel on a 125' yacht. There were 14 tourists on this trip with 2 naturalists, one of which was Tui DeRay, renowned photographer who was born and lived in the Galapagos Islands. Her mother was still living there when we were there, but not in good health.

I took way too much gear. I took 2 bodies, 300mm prime, 70-200, 24-70, and 16-35. I used a 2x and 1.4x teleconverter sometimes when taking pictures of BIF or birds that were far away. I carried both cameras on a dual strap all the time so I could switch from the 300 prime, to a zoom on the other camera body. On some islands, I knew that I'd be doing mostly landscapes so I didn't bring the large glass.

The crew is usually very helpful when dong beach (wet) landings and the dry landings as well. They don't want anyone to get hurt. They will usually help with the equipment getting on and off boats.

If I had it to do again, I'd take my new 100-400 Mark II lens and a 24-70 on a full frame body capable of 6fps. I'd also bring a 1.4x teleconverter for the 100-400 lens since they work well together. I carried prefit plastic bags to cover my camera and lens when I knew that we'd encounter precipitation or misty weather. I fit them so that they would fit over the end of the lens, use a rubber band to keep it tight near the end of lens, and cover the entire camera so that it would stay as dry as possible.

The most important part is if you plan to do the snorkeling, bring a GoPro camera on a short stick. They won't allow the long 3 or 4 foot long sticks because they bother the sea life (they say). I brought the long stick and they were constantly telling me to put it away but I was not bothering the fish so I used it whenever I was out of sight of the naturalist (police). Also, if you plan to snorkel, reserve a wet suit and a weight belt. You need a weight belt to go down 10-12 feet and stay down while doing video of the fish, penguins, flightless cormorants, seals and turtles. They are fantastic. Also, if you have facial hair, consider shaving. The masks don't make a tight seal on your face when you have a mustache. The crew provided vasoline to smear on the mustache but it doesn't work the best but it's better than nothing.

I brought a small laptop computer so that I could upload all my images and format my cards. I used a small USB 512GB external solid state drive for storage because the laptop hard drive wasn't very large.

I brought with me a wetsuit, fins, snorkel, mask, and GoPro with all my other camera gear. On the last day of the trip I sold the wetsuit gear and GoPro to one of the staff members because I knew that I wouldn't have use for them again and didn't want to lug them home. You can arrange to just rent the snorkeling gear and that is what I'd probably do if I did it again. I'd should have kept the GoPro because later I found a good use for it and had to buy another.

Good luck and have fun.

PS - carry on all your camera gear. Wear a jacket or vest with large pockets so that if you are told that you must check your bag(s), which could happen, you can just put your lenses, and bodies in your pockets and still carry them on the jet. Under no circumstances should you check your camera or lenses. Take one change of clothes and any meds on the plane with you too in case they lose or steal your checked luggage. Do not drink any water or eat anything that could have water on it except for bottled water, or boiled water, coffee and tea are ok, and make sure the ship or yacht has filtered water. I wouldn't even use tap water to brush my teeth. I learned this from my sister who has traveled extensively in the past.

Having recently been to the Galapagos Islands I concur with other readers that your kit is fine. Most animals (except flying birds) can be approached close enough even if you remain on the approved trails and areas. You might take along cotton balls or q-tips or a fiber free cloth and some rubbing alcohol to wipe down your camera/lenses after a day in the field as you will be traveling from ship to shore by zodiac type raft for your expeditions. I took many photos (dolphins, birds, penguins, iguanas, etc.) while on the rafts and the salt water spray can play havoc with electronics. I wiped down all the exposed surfaces at the end of every day. I might also suggest several extra batteries, charger with appropriate plug addapter AND small sized memory cards not the 64-128 gig type but use 8-16 gb. Why? Because if a chip goes bad you may lose thousands or all your accumulated photos with larger cards. I have seen this happen to other photographers more than once. I took enough cards for a thousand photos/day/camera and backed them up to a laptop every night.
Lastly, format your cards then take test photos with each lens and make sure your batteries will charge and discharge before you leave home .... make sure everything works appropriately before you leave home on a once in a life time $$$ trip like a Safari or Galapagos. I don't remember any camera shops in the islands that carried parts other than overpriced memory cards and aa type batteries or could do repairs.
Don't get so hung up on taking photos that you miss observing the marvelous creatures that exist around you. Most do not have any natural predators and show no fear of mankind. So cool!