A very important notation on navigational charts are wrecks and other hazards. These are represented by a variety of symbols which demonstrate the variety of wrecks that you can encounter on the water.
The dotted line is to delineate an area where a hazard exists. It would behoove you to not sail over these marked areas. Some dotted and shaded areas will have a depth mark on them, these are somewhat safer to navigate over as the depth shows the depth of safe water over the hazard.
This symbol surrounded by dotted lines indicates a wreckage which may show part of the wreck through the surface of the water. PA stands for "Position Approximate" which means they don't know exactly where the wreck is. In general, this is a good warning to stay away because the wrecked vessel could be just under the water and cause serious damage to your yacht.
Another symbol for a wreckage looks like a boat coming out of the water. This symbol refers to a partially submerged wreck where part of it can be seen above the water.
Asterisks are used to denote rocks that are awash. Rocks marked by a plus sign (+) signify a rock of an unknown depth.
In general, dotted lines signify places that you should avoid while navigating. If a depth number is marked in association with the obstruction, and you will have enough water between your keel and the obstacle, you may traverse over this area. While it might be safe to pass over, the ocean is huge and there is plenty of water around the obstruction, so why not play it safe and go around the potential hazard?