A narrow coastal body of water, a channel or strait, usually one that is subject to strong tidal currents flowing back and forth.
A small creek.
Many guts are straits but some are at a river mouths where tidal currents are strong. The comparatively large quantities of water that flow quite quickly through a gut can cause heavy erosion that results in a channel deeper than the rest of the surrounding seabed, and the currents may present a hazard to ships and boats at times.
The term "gut" is primarily (though not exclusively) applied to channels of the coastal waters of the Atlantic coast of North America. A similar term of related but not identical meaning, "gat", is applied to some narrow waterways of the North Sea and Baltic Sea coasts of Europe.