Why do they build nuclear plants next to oceans

In short to use the water as a cooling source. If you are referring to Japan it is because they are an island and everything is close to the ocean. But not all nuclear plants are next to oceans.

USA locations
http://www.nrc.gov/info-finder/reactor/

Japan
http://energybusinessdaily.com/power/nuclear-power-2/japan-nuclear-power-plant-locations-and-details/

list of all power plant types around the world
http://www.industcards.com/ppworld.htm

Nuclear plants run hot - they need a large supply of water to cool them, especially if there is an issue/emergency. Almost all currently operating nuclear power plants are light water reactors using ordinary water under high pressure as coolant and neutron moderator. About 1/3 are boiling water reactors where the primary coolant undergoes phase change to steam inside the reactor. About 2/3 are pressurized water reactors at even higher pressure.

Because they are stupid. I'm and architect. I can't believe they do it. Disaster is inevitable. In the battle between mother nature and buildings, mother nature always wins. The space shuttle assembly building in Cape Canaveral is only rated for wind up to 90MPH...Does that make sense?

Not only should they not be built on the oceans, they should be earth sheltered and protected from winds up to 300MPH and never be in any possible flood zone, fault lines etc. You can't avoid every possible disaster, but you can sure play your cards a hell of a lot better.

Japan did it becasue they are an island nation they don't have an interior from the ocean like the midwest or russia pretty much all of it is on the coast.

And stupid was a harsh word. I would say egotistical.