It depends. My thought would be to base it on tine/antler restrictions. Moose are not hard to see, but if you need to worry about counting tines or inches from a little distance, it would probably help to have one. If your in an any bull area, I'd leave it at home. I'm doing a fly in trip for moose this year but antler restricted, so ill be taking my scope setup.
If you're hunting an antler restricted area I'd take it, sometimes bino's are just not enough to allow you to get a better look at those brow tines. If the area you are hunting is flat it won't do you much good looking over the landscape but it could help you focus in on bulls that are way out there.
I agree with the above suggestions ^^^^^. In an "any bull" area I just take my 10x42's. If I have to count brow tines, then I also want my spotting scope to save me unnecessary hiking to get a better look.
I'm assuming this is an Alaska float. If so, another factor is the terrain/topography of your float trip area. Some float areas up here are flat with thick/tall willow and other shrubbery/trees lining and choking the shoreline, so visibility is limited and you won't be looking through your spotting scope the entire trip...period. Other float areas the river may wind through hill country that will be more open, so climbing a hill alongside the river may be a great opportunity to use a spotting scope.