Like any wildlife shooting whether your lens is long enough...it depends. You can at times find places along the Skagit where the spawned salmon are on a gravel bar or beach that you can approach and be well within the reach of 200mm or 200mm plus a 1.4x. Other times 600mm is not enough regardless.
The suggestion for extension tubes I would guess is a joke, as those will limit both the near and far focus ability of the lens. If you can get within 20-25' they would be helpful but at more normal distance measured is a couple dozen yards or more they would be useless.
On the floats you will see a lot of birds, many on the shore edge but many more in the trees and so you will be shooting the probverbial... "see this is an eagles rear end shot". This is not to say you might not find some good shots at or near eye level. But understand the boat is floating with the current and you need to be prepared to grab a few quick shots if they present themselves because the guide will slow the boat but rarely stop it.
You stay fairly dry and with a few precautions the camera should be fine, a dry bag is very handy to have and some plastic produce bags can be used to cover the body/lens if (yeah if! ) it rains. But the water is fairly placid but any kind of enviornemnt like this be prepared for water to be splashing about, other visitors pushing, and moving about to get the best view.
You will have a lot of fun and see some very nice birds regardless of your choice of moving along the river, but if it were me I try to find a place where first it is legal to get down to the river and 2nd where the river has a bend or break that brings the carcasses to the bank and there I sit and wait for the bird to come to me. If there are a number of birds and few fish you will get them coming close if you are patient and quietly still. But you can see a large number of birds that are away from the river as well, almost funny but if you see roadkill an eagle will be visiting it so keep your eye on it or around it they are probably eyeballing it as well from nearby.
Don't wear white and do wear a hat it really makes a difference if having the birds approach you or vice versa.
And as almost an aside, the Festivals run doesn't promise birds, it is just a convenient time for the community to hold it. The eagles can come earllier or later but will eventually show up, so if it is bust one day the next can be an entirely different story. And if the eagles aren't there do go down river to the Skagit flats & shoot some snow geese, peregrines and short eared owls among other interesting subjects which are all out there right now.
Forgot to add, the sun isn't rising until about 8:00 am, and the river is shadowed by trees on it is westerly run so a 6:00am start would not let any shooting happen for at least 3 hours and then at an ISO of 800 and wide open. Plus if you are in a boat you are going to want shutter speed as much as possible, the boats movement will have to mitigated by the fastest speed you can get, I would guess based on other water shooting I have done 1/1000th on a IS lens is a good starting point but personally I would perfer twice that or more. That is probably the biggest challenge from shooting in a raft. That said, morning is better because the carcasses that hung up over night are available, during the day these get consumed and the eagles look for other options. Have fun, good luck!
