Did you try THINKING about this instead of just looking for the answer on the internet?
After you drive home from work tonight, open the hood of your car and place your hand on the engine. After you have finished screaming and put soothing cream on your hand, you will realize an important fact: anything that produces (or uses) energy, since it is not 100% efficient, also produces "waste energy"- heat.
Nuclear power plants produce a lot of energy and so a lot of heat. They need to be cooled- normally this is done by pumping water of the core and, of course, out again. The water going out has removed heat from the power plant and carries it with it.
Actually, it's not correct to say that ALL nuclear power plants raise the temperature of the sea- some nuclear power plants are nowhere near the sea! They just raise the temperature of a local river or lake! (Which eventually flows out to sea- maybe you're right!)
But the literal answer is hiding in what that naughty mentor said. The nuclear reactors get hot, because they are used to boil water to make steam to run steam turbines to turn electric generators to make electricity for people to use. In order to keep them from getting too hot they are cooled with water - just as the engine in most cars is. The water takes away some of the heat of the reactors and so it becomes hot itself. After it is used, the water is dumped into the nearest large body of water - which is why reactors are located by lakes, rivers, or the seaside. The hot water raises the temperature of the water it is dumped into.
Mentor is right that you should practice working things out for yourself, but I am betting you didn't have enough knowledge about the problem to do that on your own. Hope this helps.
By "do you have stairs in your house?" I think meister is pointing out another example of how, when energy is transformed, some of the useful energy is lost as heat. Perhaps the mentors could be so kind as to cite and explain simply a law of thermodynamics or two.
To produce electricity coils are made to rotate in a magnetic field. The energy to rotate the coils often comes from steam. In some countries coal is burned to heat the water to produce steam. In the case of nuclear power plants, the heat comes from controlled nuclear reactions.
Water is taken from a lake or river, and heated so that it is under a lot of pressure. The pressure is used to turn turbines. As the pressure is released, the water cools, but not to the same temperature as it was when it came from the lake or river. So, the wter is returned warmer.
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