This made me LOL! 
Yeah, the more I think about it; I’m just being silly. We’ve had this arrangement for like three years now and this is the first time it’s ever happened.
I guess I’ll just forget about it.
I was going to suggest a cloaking device, but this seems a lot easier.
Post them online.
Bingo. Prevents embarrassment all around.
And type slowly.
I agree with the idea of making noise. Pretend you’re on the phone, cough, slam a door, play loud music, just something to let her know that someone is home.
Talking on the phone is good, you could loudly say something like “you should see what’s lying by my pool!” 
Was she like floating on some device in your own pool or was she on her side of the backyard area.
I think you have a legitimate right to access your own pool and ignore what the neighbors may be doing, if they have a problem with that, then it is their problem.
Surveillance camera.
Park down the street.
Remember that argument about how, at 68°, the temp of the pool is just too cold?
She wins. With Cite.
I doubt that.
That made me laugh 
You were supposed to jump in and say, “Hey look, I’m topless too!”
If it happens again and you go out there and act like it’s no big deal, maybe she’ll be OK with it too. Win-win!
I think you handled it in the right way, but I got to thinking about this. What will you do if it happens again?
This strikes me as someone who has gotten a little too comfortable with the arrangement and is taking liberties. If she wants to go topless, she should do it in her own yard. In this situation, you were put into a uncomfortable position and were unable to use your own pool because, IMO, she assumed something about the arrangement she should not have.
Also, you didn’t say whether you have kids, and I haven’t been here long enough to know if you do or not…is there a chance your kid, her kid or someone else’s could have seen her?
I think I would try to come up with some reasoning to stop the use of the pool when you are not home. This will eliminate any situations from like this from happening again.
it’s just boobs.
Duh–You should have fucked her.
This is what Facebook is for, take pics and post them. Everyone will have a good, hearty laugh.
What?
Maybe she really knew you were home.
There are boobs and then there are BOOBS!
Hopefully, Shakes experienced BOOBS!