Fit is critical, since there is not much selection where I live I had Amazon deliver 3 pairs, kept the good ones and sent two back.
The really thin ones you can have em fit tight like a glove but the sole is also thinner and doesn't provide much protection from sharp coral and rocks.
I ended up with Oneill psychofreak 3.5mm, my feet are a bit narrow and not so tall above the arch and these fit the tightest. If your feet are wide most brands will fit good. Mine are split toe, without a split toe your toes will become jammed in the front of the bootie.
These have a strap at the top and one over the arch area, I could not imagine using booties without the straps, too much water and sand would pour in, as it is they end up with a lot of water trapped in them. I have an idea to have little one way valve thingies along the sides like a scupper valve on a boat that would let the water out but not in. Even good fitting booties will sometimes fill up with water and look and feel like clown shoes.

I have used Vibram 5 finger booties, they work really well but let a lot of sand in if you spend a lot of time floundering around with your feet on the bottom of the ocean like me.