Hi Helge,
I would definitely bring a wide zoom and a medium zoom too. Galapagos (underwater) is full of big stuff, but not so big and docile that you could get by on a fisheye alone. Though it would be fun for the sea lions and whitetips (and eagle rays and whale sharks if you get to darwin & wolf). Most sharks are more timid, especially the hammerheads, and would be tough to shoot with the fisheye. The penguins and marine iguanas are small enough they would also be tough to fill a fisheye.
You may want to bring your macro setup if you are going to the western side of Isabella where you find seahorses and frogfish and redlipped batfish, but only a 60mm not a 105mm as these aren't too small (the sea horses are almost a foot long! Also Cousin's rock near the main islands is a fantastic macro dive. There are probably other macro dives too.
Also, if you have any big zooms, you will want one for topside. You will be doing land tours I'm sure and the bird life is fantastic. I brought my nikon 80-400 last time and it didn't leave my camera topside. There are some cool macro creatures topside like the crickets so the 60mm would again be useful. Again, a wide zoom is necessary for landscapes that would be too distorted with a fisheye.
Whatever you bring I'm sure you'll have a fantastic time. What a magical place! Cheers,
Chris