I use a Z600TC on a Z9 and a 100-400 on a Z8 with and without a TC for most of my wildlife shooting. For me the 100-400 provides alot of flexibilty. On pelagic trips that chum, the birds are very close, and somewhere between 100 and 400 allows you to fill the frame and not clip the wings. At 100mm it is relatively easy to acquire flying birds and fast to move to 400, and along with the smallest AF area you can get away with before handing off to Auto makes for a relatively smooth process. For birds that are sitting on the water I either use wide small or single point on a calm day. If the bird flies then a handoff happens.
On a boat that tosses bait fish in the air to feed surrounding birds such as gannets the 100-400 also works well. Action is close and fast. 100 or so also allows you to fill the frame. I also like the 100-400 on Zodiacs where people are sitting shoulder to shoulder and since they aren't chumming the action may be farther away. With a TC you can get out to 560. If you're around whales as well the 100-400 allows you to get most if not all of the whale or other large mammal in the frame should they happen to come close. Also I didn't knock a single person into the water since the lens is small. Light is not often a problem out in the ocean so shooting at f5.6 or 8 works well.
I was recently on a cruise ship in the Arctic off Greenland and Norway. The ship had three decks. The most numerous photo opportunities came on the upper deck where most of the photographers and birders gathered. Lots of observers and the best field of view. Here I used the 600TC. Whales, dolphins, and seals were not right on top of you and many were far away. At 840 I could get pictures that others couldn't. I could have, but didn't use an extra TC. I also spent time at the sides near the rear on the lower deck so I could move quickly from port to starboard. Our stateroom was at the stern so if I saw something from the room I could access the rear deck quickly. The 600TC worked well here too with the seabirds that were flying by and taking off as we approached such as auks, fulmar, kittiwakes, puffins, and guillemots. Some fulmar were too close for the 600, but since there were so many it didn't matter.
https://mrtk.smugmug.com/Cruise-to-Svalbard-528-6624/i-mCpQntD
In a kayak I like the 100-400 because it fits easily in the cockpit on your lap and with the Z8 is light and is easy to bring to bear thanks to finding flying birds at 100mm and moving to 400. You can also get relatively close to birds both on shore and in the water. In a canoe I bring both cameras and lenses in a dry bag.
So all in all I really like the Z100-400 in boats for so many reasons that I'm glad I went that route. As so many people suggest, you may want to rent those lenses for a few days and shoot in a variety of situations if possible.