A float hunt opens up the opportunity to cover such a large area. You can float for an hour, glass and call for a bit, and move on if nothing presents itself. I wouldn't worry about the dates. If you're getting dropped off by an aircraft, early September is generally a pretty good time to be flying around, anything after the 15th can get pretty unpredictable due to frequent cold weather systems.
Where I worked in western AK we hunted the non-resident season and generally didn't see a ton of moose until the first hard frost, usually sometime around September 7-15. That doesn't mean we didn't kill any on the first day of the season, but it was really up to you to get out and find a meadow that had a bull hanging around. We hunted out of a lodge setting with a few out camps strewn about a few different areas, so we didn't necessarily have the luxury of pulling anchor and floating 10 miles downstream to a different drainage. Early season is less about calling, especially cow calling, and more about just listening and looking, maybe doing some raking. One of my buddies brought one in after we snuck into his digs one morning, raked just a little, and then filled his moose tube with water and let it dribble back into the marsh we were standing in. He said "that boy thinks we're pissing on his turf". Ten minutes later we were putting the locking tag on him.