Well-fitting, well-constructed shoes.  The strap is great for me, with my narrow feet, keeping the shoes hugging them instead of flopping around or rubbing.  Good traction on the pool bottom, no problem on the deck.  I'm praying they last a long time, because I like them so much!
I chose a size 9 for my size 9 1/2 feet, and that was the right thing to do.