The weather, tides, currents and water depth can all change quickly so it is important to be aware of these before you enter the water and when you are in it. Even in summer, the water can be extremely cold, leading to cold-water shock. It may stop you from swimming and may affect your breathing.
You may not be able to see underwater dangers which may hurt you, especially if you dive or jump in. The water may also be used for other activities including sailing, kayaking, jet-skiing, or possibly industrial or farming activity. Check before entering the water.
Sea-life such as jellyfish or weever fish may also be in the water and are best avoided. If you do get stung, most stings are not likely to be serious and can be treated with first aid but sometimes you might need to go to hospital.