Apeks RK3 Fins Designed in collaboration with the US military and renowned for outstanding performance and reliability, the Apeks RK3 is a rugged thermoplastic rubber fin featuring an over-sized foot pocket to accommodate dry suit boots and a spring strap for easy donning and doffing. The RK3 is available in three sizes of medium, large and super. Apeks didn't give the RK3 fin a shoe size range as this can be very misleading. The environment that an RK3 is used in will tend to dictate the size of fin that you need, for example do you dive in a warm environment wearing wetsuit boots, or are you walking across pebbled beaches and wearing a dry suit and rigid dry boots? Both forms of footwear may be a size 9, but will take up different amounts of space in the fin foot pocket. Therefore, Apeks recommend that you visit your local dive store and try the fins on with your own footwear, just to ensure that you're getting the right fit for your style of diving.