Apeks RK3 Open Heel Dive Fins

The RK3 has been standard issue for military, special ops, coast guard, and public safety teams for decades: a rugged thermoplastic rubber fin with a short, wide vented blade that delivers power in tight spaces and holds up to abuse that destroys composite fins. Stainless spring straps are standard, with grommet holes for clipping the fins off when climbing a ladder.

The HD is molded from a higher-density compound, which makes it stiffer and negatively buoyant — Apeks positions it as ideal for wetsuit diving, where sinking fins help keep your legs in trim. The standard RK3 is the softer-flexing original that most drysuit and all-around divers choose. Same blade shape, same spring straps, same near-indestructible build.

Apeks deliberately publishes no shoe-size chart for the RK3 — they consider it misleading, because the right size depends on your footwear. The oversized foot pocket comes in Small, Medium, Large, and Super, and is designed to swallow anything from wetsuit boots to rigid drysuit boots. Tell us your shoe size and exactly what boots you dive, and we will put you in the right pocket.

Open-heel fins have an adjustable heel strap and are worn with dive boots, which protect your feet on shore entries and add warmth. Full-foot fins slip on like a shoe against bare skin or a thin sock, and are typically lighter and easier to pack. Most scuba divers choose open-heel fins with boots, while full-foot fins suit warm water, snorkeling, and pool use.

Open-heel fins should be sized around the boots you will actually wear, snug with the strap set so your heel stays seated and your foot does not slide forward in the pocket. Full-foot fins should fit like a firm shoe, with no heel lift, no pinched toes, and no gap that lets water pump in and out. Too loose causes heel blisters and lost fins in current, and too tight causes foot cramps.

Choose your boots first and size open-heel fins around them, because sole thickness changes the foot-pocket size you need. A thin warm-water boot and a thick cold-water boot with a rugged sole can easily be a fin size apart. If you already own fins, bring them along when you buy boots so you can check the combination.

Spring straps replace the adjustable rubber strap with a stainless spring that stretches on and off, so there is nothing to buckle with cold or gloved hands and no rubber strap to perish and snap mid-dive. They hold constant tension, which many divers find more comfortable and consistent. They are sized rather than adjustable, so match the length to your foot pocket and the boots you dive.

For travel, length and weight matter most, so shorter blades and travel-specific open-heel or full-foot models pack more easily and save baggage allowance. Some fins have removable blades or compact foot pockets designed for luggage. Pack fins along the outside walls of your bag, where they also act as protection for softer gear.

Paddle fins use a solid blade that transfers power directly, which many divers prefer for precision, control in current, and non-flutter kicks such as the frog kick. Split fins have a slit down the blade that lets water pass through, which generally makes flutter kicking easier on the legs but gives up some control and works less well with other kick styles. Neither is inherently better, so let your kick style, leg strength, and conditions decide, and try both if you get the chance.

Rinse them in fresh water after each dive day, paying attention to buckles, springs, and strap channels where salt collects, and dry them out of direct sunlight. Store them flat or hanging from the foot pocket rather than standing on the blade tips, which can take a permanent bend. Check straps, buckles, and any screws before every trip, since a failed buckle is the most common fin problem.

Yes — the oversized foot pocket was designed to swallow everything from wetsuit boots to rigid drysuit boots, which is exactly why the RK3 is standard issue for military and public safety teams. The standard RK3 is the softer-flexing version most drysuit and all-around divers choose; the RK3 HD is molded from a denser compound, so it is stiffer and negatively buoyant, and Apeks positions it for wetsuit diving where sinking fins help keep your legs down. Tell us your shoe size and exactly which boots you dive and we will put you in the right pocket.