These areas might be used for sports, by commercial fisherman, or if there are signs of the area having been used for baiting or feeding areas - diving seabirds are good indicators. Sighting of Porpoises does not indicate that sharks aren't around. Often, both eat the same kind of food. Use extra caution when waters are murky. Avoid uneven tanning and bright colored clothing.
Sharks see contrasting colors really good.
Refrain from excess splashing and do not allow pets in the water because of their quick movements. If you get close to a shark, do not annoy it.