It seems to me that you'd need mesh or rebar for, if nothing else, a tie into your pool shell bonding. Having said that, since you have freeze-thaw considerations, you want an air-entrained mix with chert free aggregate. Tell the contractor you want the same mix your state DOT specs for sidewalks and other flatwork. That will cover the above proportions needed for a quality mix. Moving on to slab reinforcement, you will want a wire mesh centered in your slab. They will tell you they pull it up as they pour, but that just doesn't work in the real world. Try standing on some mesh and pull it up at the same time. Doesn't work does it? The next consideration is the design of your control joints and expansion joints. Concrete will crack, but you can determine, to a large part, where those cracks will occur. I'll post you a link to some info in this regard.