Foul

1. Having freedom of motion interfered with by collision or entanglement; entangled; the opposite of clear. For instance, a rope is foul when it does not run straight or smoothly, and an anchor is foul when it is caught on an obstruction.
2. A ship′s bottom is foul when it is overgrown with marine life such as barnacles.
3. An area of water treacherous to navigation due to many shallow obstructions such as reefs, sandbars, rocks, etc.
4. A breach of racing rules.
5. Foul the range: To block another vessel from firing her guns at a target.