Open heeled fins are a compromise solution...they allow a dive shop to stock a relatively small number of fins ( that the shop has to pay for ahead of time) and still be able to fit most divers--closer to a one size or 2 size fits all concept. Beyond this, they are a "real" solution for divers who absolutely MUST be able to walk on sharp rocky surfaces, just prior to getting in the water, meaning they need dive boots which they can walk with, to go into the fins...unfortunately, there is a huge waste of power transfer from the diver's leg, to the fin, in this type of a solution... Imagine a marathon runner cutting the heel off of his $300 running shoes, and using a strap to hold them on
If you are diving in warm water, off of a boat, you want a full foot fin, and neoprene sock style dive booties. Fins with good full fin pockets will fit only a narrow range of foot sizes, and will actually "fit" the diver's foot, allowing good transfer of power. Some full foot fins accomplish this far better than others.
Price will often hint at this--don't expect $30 snorkel fins to be worth anything but garbage can lining....and most freedive fins starting at $120, will have excellent foot pockets.....and then, there are the top freediving fins, models like the C4 Mustang, that actually has a left and right foot pocket, very much like running shoes, and spectacular power transfer ( they cost more than most scubaboard members would spend, due to the carbon fiber blades--they actually are awesome for scuba, but instead of giant striding in tot he water, you have to dive head first, or, you can back flop in--you can't over flex the blade on the high end carbon fiber fins)....
There are also traditional---non-freedivng fins with full foot pockets, that are good fins, but none are as good as freediivng fins!
Regards,
Dan Volker
p.s.
Oh, and there is Force Fin, with an entirely different approach to the foot / leg power transfer, and they are actually good also---see Force Fin as good, split fin as bad
