Fins need to fit your feet "perfectly"...much like with a ski boot, for you to get control and good power transfer from legs to feet to fins, the fit needs to be exact. Many people need orthotics to keep the proper posture of their feet, both to prevent cramping by supporting a high arch, and to allow the foot to be a powerful lever.
You can put orthotics in neoprene socks, and obviously they are easy to insert into booties.
Also, most divers have not specifically trained the muscles used in diving, for the work they will do in kicking fins. One excellent way to do this is by riding a bicycle...ideally with the seat up high enough that you need to keep your toes pointed all the way down at the bottom of the pedal stroke...and you attempt to drive the pedals forward, initially with your glutes for an instant, but then with your quadriceps for the majority of the upper part of the pedal rotation...and while one leg is doing this, the other foot is pulling straight backward on the pedal, like you are trying to scrape **** off of your shoes...sorry for the expression--but it is commonly explained this way in cycling. This push forward on top, and pull back on the bottom, combined with the toed pointed maximally at the bottom, is perfect conditioning and almost exactly the same work you do in fin swimming....
My husband, Dan Volker, uses this training form on his bike, to keep his fin swimming muscles so well trained that he can easily swim at the speed of $4000 scooters, for up to an hour at depths as deep as 80 to 90 feet. That's about 4 times the speed of a normal scuba diver, with air consumption that lets him do these dives for a whole hour--meaning he is not working that hard...and certainly not cramping....Dan DOES wear orthotics for all of the fins he uses, and he suggests a kick stroke with most of the bending being just at the knees..not much from the hips at all, as the legs cause too much drag when you use a high amplitude leg kick. And his suggestion to you would be to get someone to let you borrow/demo a pair of Force Fins....which focus the muscles in your leg you already use, and do best with a knee kick with oscillation causing speed when needed. Dan can swim at scooter speeds with freedive fins, and with Excellerating Force fins....no way with Jet fins or traditional low tech scuba fins.
This video was for kids learning snorkeling, but it is equally valid for new scuba divers..
