While I agree completely with Dan Volker when he says to relax and not try to go vertical, I think what you want is assurance that you can go vertical if you feel the need. I think you will have no difficulty with that if you are comfortable in the water and can swim, even a little. It will involve a little kicking, but with fins it won't take much, and maybe some swishing of your hands.
I often go vertical while snorkeling in order to see above the water: where is the boat, where are the other snorkelers, etc. But it's just a brief glance, and then I'm back horizontal looking at all the cool stuff.
I also second the suggestion to take a freediving class or private lesson. The fun of snorkeling is greatly magnified if you can learn to swim down a few feet. Performance Freediving International (where I learned freediving) offers classes in Ft. Lauderdale and Miami. They will teach you how to hold your breath, how to equalize your ears, and the physiology and mechanics of freediving. Of course, just spending a lot of time in the water will make you more comfortable. It sounds to me as though your only problem is uncertainty how you will feel in water too deep to stand up in. But when you realize how easily you float, I think this fear will dissipate.
I often go vertical while snorkeling in order to see above the water: where is the boat, where are the other snorkelers, etc. But it's just a brief glance, and then I'm back horizontal looking at all the cool stuff.
I also second the suggestion to take a freediving class or private lesson. The fun of snorkeling is greatly magnified if you can learn to swim down a few feet. Performance Freediving International (where I learned freediving) offers classes in Ft. Lauderdale and Miami. They will teach you how to hold your breath, how to equalize your ears, and the physiology and mechanics of freediving. Of course, just spending a lot of time in the water will make you more comfortable. It sounds to me as though your only problem is uncertainty how you will feel in water too deep to stand up in. But when you realize how easily you float, I think this fear will dissipate.