Sea Trap

A Sea Trap (シーローラー, "Shirora") is a barbed, double wire held in place by two sea urchins that infrequently surfaces upon the sea of the World of the Ocean King, in Phantom Hourglass, while Link is traveling from island to island on the S. S. Linebeck. Although it is possible to attack the two urchins that bind the wires with the cannon, it is much easier to just boat-jump over them, in a similar way that Epona does in terms of horse jumping in Ocarina of Time and Twilight Princess, on solid ground. Since they're immobile, it's also an option to stop and change course to avoid them completely.

Trivia

 * Sea Traps are the only enemies Linebeck directly explains how to defeat. Because of the explanation scene, a Sea Trap will always be the first sea-borne enemy Link encounters in Phantom Hourglass.
 * Before Link grabs the Cannon on Cannon Island or leaves the Southwestern Sea, Sea Traps are the only living enemies that will appear on the sea.
 * Sea Traps are the only enemies which can be completely avoided by boat-jumping in Phantom Hourglass.