There are recommended daily intakes of everything things your body needs including water, sodium, chloride, potassium, glucose, protein, vitamins etc. If you become dehydrated you have been losing mainly sodium and water, therefore both need replacing. Sea water has too much salt, too much salt damages your kidneys among other things.

When enough salt (sodium) is given, other intravenous fluids such as dextrose fluid (sugar water) is given, the limits for sugar are much higher than salt so it doesn't matter so much.

Additionally, giving pure water intravenously is not a good idea because if it doesn't have enough electrolytes or molecules like sodium or dextrose it can have an osmotic effect i.e. Water will move into the cells rather than stay in the veins where it is needed. This can swell and damage organs. Drinking pure water can cause this too but its less likely.