The principal criterion I apply to fins is that they fit properly, not only in terms of foot length but also foot breadth. This particularly matters in the case of snorkelling when swimmers may wear fins barefoot or with a thin sock. The German Standard of October 1980 for swim fins (DIN 7876) focuses exclusively on the dimensions of fin foot pockets. I wish there was more data about foot pocket sizing to inform choice and purchase. It's no good recommending particular brands of fins irrespective of the sizing issue. That's like jumping to solutions before defining the initial problem properly. Everybody's feet are different and foot pocket dimensions vary widely.
I'm a snorkeller, and I believe that it's the aquatic equivalent of walking in the country, where you can dress any way you feel comfortable. So any fin that fits constitutes potentially appropriate footwear. My own preference is for classic rubber full-foot fins. I've just purchased a Japanese-made pair of Gull Hard Mew fins to snorkel with off the North East coast of England:
I learned to snorkel in the early 1960s using this classic Cressi-Rondine style of fin and I've never seen any reason to change. Fins like these are multipurpose - they can be used for swim training and Hard Mews are the fins of choice in New Zealand for underwater hockey.
Some snorkellers wear surfers' fins, such as Churchills:
This was Owen Churchill's original pre-World War II design and to the best of my knowledge, they still work!
So the search for a "specialised" snorkelling fin, I believe, will be an elusive one and I take comfort in that. I know this contradicts modern diving theory which claims that there's a specialised item of gear for every aspect of diving. If you're really looking for the specifications of the ideal snorkelling fin, I suggest you create a seventh category for freediving fins. Freedivers set a lot of store by lightness, power and speed. I'm a snorkeller, not a freediver, and I don't require those attributes when I snorkel.
