One embodiment of improved swim fins consists of a right foot attachment (101) with an outer right blade (111) and an inner right blade (112) which is arranged such that it does not mechanically interfere with an inner left blade (113). The blades may include a pivot mechanism (220) that allows the blades to maintain an optimal angle of attack as the swimmer moves their legs up and down. In another embodiment, a right blade (1011) and a left blade (1013) are attached to the feet of a person who is propelled by the blades above the surface of the water as they pump their feet up and down, forming a human-powered hydrofoil. Other embodiments are described and shown.