Fins are very personal items of equipment, not least because they have to fit your feet, not somebody else's. It's wise to begin with your foot dimensions rather than a specific brand, whose foot pockets may suit your feet lengthwise but not in terms of width or height. Fins are only sized by foot length and each model is designed to cover a range of sizes, e.g. UK 10-12, not a single shoe size. The width and height of their foot pockets also vary from model to model, so make sure you try them on before purchase, preferably in the water. If your foot pockets are too loose, they will raise blisters or fall off. Too tight and they will generate cramps. Fit fins the same way as you would do a pair of shoes from which you expect years of comfortable use.
I have several pairs of fins and it has taken me years of searching to find ones that fit my long, broad feet comfortably. If you can't locate a pair in a size that fits your feet exactly, buy a pair that are slightly too big and wear fin socks, boots or booties for added bulk as well as warmth and protection. Wear this footwear when you first try on your fins to make sure you have a comfortable fit. Fin grips, three-way rubber straps worn over the ankle, heel and instep, can be bought to secure full-foot fins that are slightly too big:
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Another method with slightly over-sized full-foot fins is to use insoles:
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Click on the above images to get a closer view. Only after you have solved the foot-fitting issue should you proceed to the other criteria, such as fin type, material and flexibility. Much will depend on your strength, stamina and the kind of snorkelling you intend to do. Fins are worn for power, manoeuvrability or endurance. Despite claims to the contrary, few if any single models combine these characteristics in equal proportion. As a snorkeller, you have a wide range of choice when it comes to fin type, you don't have to follow the scuba diver's preference for open-heel fins or the freediver's commitment to long-bladed full-foot fins. For what it's worth, I snorkel here in the UK with old-fashioned, comfortable, rubber full-foot fins because that's the kind I started with back in the 1960s, that's what I remain happy with and I see no advantage in changing to more "modern" fins made of so-called "space-age" materials just for the sake of it. Of course, that doesn't mean you should follow my lead. Tracking down fins that suit you, not anybody else, is a personal odyssey and nobody can shorten the operation for you by recommending particular brands that suit a dive store salesman who may be more knowledgeable about scuba or freediving than snorkelling anyway.
Good luck with your quest for the perfect fins that accommodate your feet comfortably and suit your style of snorkelling!