I have done exactly that. I will stick with my recommendation except add full foot Mares Planas. I have limited experience with free diver fins but I am sure they are fast and will be getting some for my Florida Fling this coming summer for current diving.
As a swimmer, you use your arms a lot, for a significant amount of your power. SCUBA is just the opposite in that you do not use your arms at all! All movement, maneuvering, front, back, whichever should be done with your fins. This way you can carry cameras, lights, tools. Hand swimming is bad, bad, bad and immediately identifies beginner and untrained, undisciplined divers.
I too am (was) a competitive swimmer and open water racer. I am somewhat amused at all of the divers who use "SCUBA" and "relaxing" in the same sentence. Relax my axx, I will relax when I am dead, as long as I breath, it is go time. But the real use for swimming is fitness, aerobic capacity, water safety and water confidence.
N