Normally in a landscape or seascape shot, the horizon is kept level. However, I sometimes shoot scenes in which the horizon is naturally slightly slanted, such as tidal inlets, bays and other bodies of water. If I were to enter such a shot into a competition, should I leave the horizon slanted, or use Photoshop to straighten it out?
Some may well allow it. Some also will expect it. In the end, it is up to you. You may deliberately choose to slant the picture, and that is part of the art of photography. If you wanted it that way when you took it, submit it that way. At the very least, it will stand out from all of the other one's with perfectly straight horizons.
To me it is what looks best, some of the shots I have taken I have experienced what you have mentioned however straightening the horizon can ruin the rest of the image. Trouble is that some people will just pick up on an un-level horizon without looking at the rest of the image. Also it is not like you can say in a competition yeah the horizon was naturally un-level here it is with the horizon straightened.
Having said that if the judges of a competition are worth their salt and the image is well composed they should be able to tell the horizon is naturaly unlevel by looking at the rest of the image.
If you shoot on a tripod with a spirit level in your hot shoe at least you can be confident the camera was level when you took the shot!