Drinking a little bit of seawater is not actually particularly dangerous in itself. If you have ever been swimming in the ocean you are bound to have accidentally done this and you are still here. However you cannot hydrate by drinking seawater, and in fact you will die of thirst quicker by drinking seawater than by drinking nothing.
At the same time seawater in large quanities causes tremendous strain on your organs. This has happened to many sailors in the past, and is a sadly ironic way to go, dying of thirst yet surrounded by useless water.
The reasons for seawater being this dangerous or useless are simple. Seawater contains more salts than are present in our bodies. To give you an indication of what we contain, try one of the rehydration drinks you should drink when you have a serious stomach upset. This contains the same balance of water and salts present in you. Seawater however has an awful lot more salt than this.
Now too much salt is extremely dangerous to our bodies and we have to get rid of the excess. The only way we can do this is by expelling fluids along with the salts. Salts come out in tears, sweat and urine but we cannot excrete salts by themselves or with only a small amount of water, although many animals adapted for life in the oceans can.
In the effort to get rid of the excess salt, which is toxic to our cells if allowed to remain, the kidneys go into overdrive and may collapse. Even before this happens more water is being expelled than is being taken in. The dehydration you were drinking to avoid is itself speeded up.
The final danger is that after your first drink the effects on your body affect your mind and all sense of judgement goes. You might have started off thinking one glass of seawater would not kill you and it would relieve a parched mouth. After drinking it you might well forget that you didn't intend to follow this with several more glasses, which would be toxic.
There is a small lesser danger to drinking seawater and this is contamination. Theoretically you could drink a little seawater safely each day if you also drank a lot of plain water. However you are taking in more than just salt and water . You are also taking in any pollutants or chemicals that have been dispersed in this area. Over time these will add up and you will get sick. This happens in many marine animals. Tests have shown that levels of for example mercury are exceedingly high in turtles and fish.
But don’t despair. Should you ever be in the situation where the only water available is seawater you can turn this into fresh water by distilling it. This can be done either by heating it and collection the steam or more slowly by allowing it to evaporate and catching the evaporation. The salts are left behind and you will get nice fresh water. It takes time but it would save your life.