Complex sentence: The rip is like a moving treadmill, which the swimmer can get out of by swimming across the current, parallel to the shore, in either direction, until out of the rip current, which is usually not very wide.
Split into simpler sentences: The rip is like a moving treadmill, which the swimmer needs to get out of. This can be done by swimming across the current, parallel to the shore, until out of the rip current, which is usually not very wide.