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Single-sided
Often called “dipping” magnets, these attract treasures more effectively when dipped up and down off the bottom of a body of water. They’re ideal for dropping off of docks, bridges, and piers in slow-moving water. When lifted, single-sided magnets retain an angle that promotes maximum pull force (because their calculated strength is all on one side), making them better for pulling up heavier objects, like a safe.
Double-sided
Best suited for steep inclines and fast-moving water like streams and rivers, these attract objects on the top and bottom of the magnet. Pull force is calculated by combining the strength of the top and bottom, so a 1,200-pound double-sided magnet has a pull force of 600 pounds per side.
Clamp
Also ideal for dragging the bottom of a fast-moving body of water, these use one solid magnet secured in a frame, as opposed to two thinner ones like those on the top and bottom of a double-sided magnet. The result is a larger magnetic field (all sides except where the clamp is attached) capable of picking up objects from farther away.
360
With a shape similar to single- and double-sided magnets, and the larger magnetic field of a clamp, a 360 is the most versatile—and typically most expensive—of the four. Able to attract objects from all sides, a 360 magnet can be used for everything from dragging in fast-moving water to dipping off bridges and piers.
Source: https://www.popularmechanics.com/adventure/outdoor-gear/a39679643/everything-you-need-to-get-started-in-magnet-fishing/
