The first question I always asked customers looking for fins when they walked into this or that dive shop where I worked was, "Do you have or ever had knee, ankle, or hip problems?"
There's a dogma among tech divers that you need to use a very specific fin. And never, ever, ever, ever a pair of split fins. Which are worthless and weak and for jerkoffs and dumbasses.
Unless - I've always thought - you have had knee, ankle, or hip problems. In which case they're pretty fucking handy. Because they put far less stress on those affected joints and can, potentially, avoid any further harm to them.
Granted: if you are using split fins there are going to be discrepancies. Alternative kicks are going to be difficult, if not impossible. Which means that, in silty, overhead environments, there may be a limitation to areas safely available to a diver. Personally, my stance is: fuck split fins. I used them once. For about 2 minutes. I'd traded my own fins with a friend who swore by them. I immediately got bored with them and removed them, floating in one spot and waiting for my buddy to return with my own (real) fins. That said, we've had guests who use them quite happily for long dives. Some people just like them. And that's OK. I've been going on about split fins for longer than I really meant to. That was supposed to just be a jumping-off point. What I really wanted to address is an eccentricity I've noticed among some divers. Usually divers who use floppier fins. Definitely split fins. Even fins with channels or softer blades. Generally divers who don't use... shall we say... something designed within the last 50 years. Most frequently, it's wreck divers. They frog kick with one foot and flutter kick with the other. It's the weirdest, damned thing. Took me a while to figure out the why and the how. But at some point I noticed that some folks have a tendency to cock their legs into the frog-kick "launch" position... And then pull one knee in sooner than the other. So one ankle cocks to perform the frog-kick. But the other knee is already so medial that ankle CAN'T cock and frog kick. The only physiological option is to drag that leg along into the medial position... resulting in a flutter kick. Which torques the diver. And has to be corrected. Which eats into energy. To the detriment of a resting, hydrodynamic glide. And directs enough water downward (as opposed to backward) to cause a silt-poof here and there. This tends to be (but not as a rule) the non-dominant foot. The best guidance I've got for folks who may be award they're doing this: The strongest moment of your frog-kick is at the end of the kick, as your fins come together, not at the beginning. Don't think of it as a wind-up and a pitch. Concentrate on your knees more than your feet. And glide like a lazy bastard before moving your legs again. For those of you using split fins... your frog-kick is probably going to suck no matter what as those floppily-dopplies waggle around behind you. Use real fins. Unless you've got knee, ankle, or hip problems. Then use whatever is most comfortable. You might have to stay out of small, silty places; but there's no rule you have to go there anyway. And if anyone tries to talk you into something else you can tell them to zip it.