What liquid to use in an airlock
I use Starsan since I already have it ready from cleaning everything else, but have used water when I forgot to save some. I've never had anything go funky either way.
If you're worried about sanitizer losing effectiveness, you can change it out wherever you want, but it's always going to be cleaner than water with no sanitizer. I just change it when I rack.
Dish soap just seems like a way to end up drinking soapy booze.