I would highly recommend the Zhik P3 PFD. It's ISO certified, but not as bulky as a USCG approved version. I had a Mystic impact vest, but decided I want real floatation just in case something ever happened. This vest is the best of both worlds. I find it more comfortable than the mystic, it has real floatation, I find it provides more impact protection, it has a useful pocket, and it's cut allows plenty of room for a harness.
Unless you're in breaking waves, I don't know why more people don't use real floatation. It could save your life.
zhik.com/us/p3-pfd-black.html