I added the yeast to my fermentation bucket and closed it up without stirring. Remembered about 8 hrs later; opened bucket, lots of yeast floating on top, stirred it up good and closed it. Have I ruined my batch?
No. There was no need to stir, however; once the yeast become active, they'll start moving throughout the wort (and getting it to move quite a lot) all by themselves. As well, stirring the wort could introduce contaminates or oxygen from the environment.
But there should never be a need to stir, either. If you're using dry yeast, you really should rehydrate it first, then pour the liquid into the wort. If you're using liquid yeast, with or without a starter, it's already liquid.
As already stated above, your batch should be OK.
For the record, the instructions on my dry yeast packets (Safale/Saflager) read:
PITCHING: sprinkle into wort. I have done this for over 180 brews with a 100% success rate. One key word - "sprinkle", so to avoid the risk of clumping, don't chuck or dump the yeast, sprinkle it. No need to rehydrate, even though it is recommended by many commentators.
Doubtful you ruined it. As long as you practiced good sanitation.
I'm guessing it was dry yeast if it was floating. I would swirl the fermentor to make a whirlpool with the fermentor sealed until it's mixed in. This should remove any yeast stuck to the sides and help get some oxygen in the wort.
In the future hydrate your yeast before pitching.
It shouldn't be ruined, the yeast should still activate and work its way through your wort without the need to stir. As a previous answer mentioned, stirring your wort at this point in the process isn't a good idea unless you've made sure your stirring instrument is very well sanitized, and even then you still would be introducing oxygen to your batch which brings on a pretty prevalent cardboard-y taste to the finished product... believe me, I've been there.