Pretty much every kid who's ever been in the water can do the old dolphin impersonation trick. Arms at the sides, legs together, body undulating. Unfortunately, it's difficult to make much forward progress.
If you really want to swim like a dolphin, you can get transpecies surgery or you can pick up the the DOL-Fin Orca Mark 2. (No, I wasn't aware of the Mark One, either.) It's a monofin swim accessory designed by aerospace engineer Ron Smith that replaces the traditional set of two swim fins.
Dolphins Call Each Other By Name
The DOL-Fin, which stands for Dynamic Oscillating Lateral Fin, leverages both material science and hydrofoil propulsion tech to approximate the action of a dolphin's tail flukes.
The fin is similar to existing flexible rubber monofins, but places value on function over form. Made from rigid aluminum, the DOL-Fin looks more like an airplane part than a dolphin's flukes. But that's entirely by design, according to Smith, because the device is made to work with human anatomy, not dolphin anatomy.